Because it's not fair to expect users to go through the effort of manually translating people's posts, and that's doubly true when it comes to the moderators. It's not like this rule is unique to this place either. Given that the majority of us speak English as a first language, and I'd wager the majority of us are also monolingual too, it makes sense to continue doing things the way we always have done. It's not like there are any shortage of online communities where you can speak Japanese with likeminded people attempting to learn.

There is a pretty good reason english is the only language. It would be a hellish nightmare to moderate, every language would probably need a moderator, and no translating wouldn't work. I can get 50 ways I can make the translator misstranslate the word I really ment in arabic, who knows about other languages.

I think I’m the only one who’d vote for this though.

Truth is, this topic was partly inspired by another sort of related topic several headings down from this one where people started speaking Sweedish. It's also partly inspired by the fact that there aren’t really any other accessible gaming communities where English is the primary language, much less forums where it isn’t. I think there’s a Spanish comunity and maybe an Arabic community, and no more to my knowledge. It doesn’t help matters that absolutely anyone who makes a new forum of any kind has their proverbial head torn off nowadays. Sometimes it makes sense, like when a kid starts a forum for kicks, but what’s gonna happen when someone who’s older with a vested interest in both Japanese and gaming comes along and tries to make a forum based around that? Cough cough.

Yeah, this is primarily a moderation issue. You see, at least for me, and probably the other moderators as well, we parse posts that may need parsed word by word and take into account context. If you speak the language, this is pretty easy. You generally understand most contexts and significance of each word. If you don't speak the language, it's a lot easier to miss things, or to misconstrue something culturally/linguistically. The results could be quite bad.

1. You smart guy I have a question for you. I speak Kurdish. And it has been added last year or two years ago I think to Google Translate. And do you know when a language is being added the first five or ten years it's like shit you can't translate anything from it. Then how you would understand me?

I disagree with this proposal because it would make it incredibly difficult to moderate on the admin/mod side. Not to mention the fact that allowing people to speak any language can cause conflict on it's own, and this forum handles enough of that as is. How does somebody who doesn't speak the language your communicating in know that you aren't insulting them, generally being an ass, etc.? You might say "well couldn't they just translate it?" but: (1) that requires that a good machine translation system be available already; and (2) I'm pretty sure there are some things in every language that do not translate well, if at all, to others, because the words/context/cultural-isms aren't available. This would be nice if it was possible, but the forum would pretty much need a complete remake for that to work, including automatic language detection in every post and automatic post translation to the users language. Most likely you'd also want separate forums/categories for different languages too, and mods for each language. Given how much the mod team struggles right now (and I don't mean for that to be construed as an insult, it's just a fact), I significantly doubt it would be wise to try anything like this now.

I adhere to @13 and @14 considerations.
While I am my self a native Spanish speakers who understands and can write somewhat in English as a second language, I think your proposal is not a good idea; not at least if there are sub-forums for each language such as the Audacity official forums.
Allowing anyone to post in his/her native language would considerably difficult the knowledge, opinion and material travels that this forum, despite its many issues, allows between so many users of different cultures that share the common interests in audio games.
This is not only due to moderation being harder in that case, but especially because automatic translation, despite how much it has evolved in the last years, still has bottlenecks, especially between languages of different families. If you were a native speaker of some language other than English, you would understand in practice the fact that often machine translations are hard to understand because sometimes generate non-grammatical sentences or choose words that are not applicable at all in context.

Unfortunately, Microsoft in recent years is using machine translation to translating user interfaces of star products like Office and Windows, and especially the help/support content. Much more often than it should, I'm in need to switch the help page language back to English to understand a given topic because the automatic translation miss information, mix UI terms or even leaves examples unadapted based on descriptive information that the machine decided to translate.
A good help example of this machine translation tragedy are topics about Excel functions, or the ones that explain how to do certain things with a screen reader, where often hotkeys that are translated in the product leave untranslated in these help topics.
A couple of UI examples will make you an idea of the difficulties automatic translation could present in the forum.

• In certain Windows 10 release I don't exactly remember which one, Microsoft added a setting to specify how many open Microsoft Edge tabs to show when using ALT+TAB in addition to traditional windows. In the Spanish version, options said things like "5 pestañas y abrir las ventanas", "Abrir ventanas y 3 pestañas", etc. Although I remembered to read about this new feature when they announced it in a blog post about an Insider Preview build, at first the setting in Spanish is not understandable, because without reading that blog post, for a casual user the translation gives the idea of showing something to open windows as an action. It happened because English, unlike Spanish, sometimes uses the word "open" both as an action ("I want to open the window") and as a participle ("The window is open"), and the machine chose to use for "open" the Spanish word "abrir" which is "open" as an action, rater than "abiertas" that is the plural participle that was correct in this context.

• In Office 2007, Microsoft added a context menu to the status bar of programs like Word and Excel, which allows to select which items we want the status bar to have. One of these is "View shortcuts". In Spanish it was translated as "Ver accesos directos", which is "view" as an action ("accesos directos" is the Microsoft translation for "shortcuts"). As a result, I thought this option enabled or disabled the possibility of clicking the items for opening dialogs such as Find and Word Count. But it actually adds and removes the buttons for changing the document view, such as Print Layout or Draft, in which the word "view" is a noun. Therefore, it should have been translated as "Accesos directos a vistas", where "vistas" is "view" as a plural noun.

• In my Nokia E5 which had Latin American Spanish, the Maps application had a setting that said "Posición inicial" (literally "Starting position" or "Initial position") and was required to use the feature for quickly traveling to our house. However, this didn't actually asked the starting position, but the position of our house itself. My confusion was because they translated as "Posición inicial" the original "Home position". While "home" is a valid English word to denote a reference or starting point for something, it also refers to a house, like it was in this case, so this should have been translated as "Posición del hogar", "Posición de casa" or in any way in which the equivalent used for the word "home" could be interpreted as a place in which someone lives.

• In Nero Wave Editor, which is the program of the Nero suite used for audio editing, there was an item named "Edit History" in the Edit menu. In Spanish it reads "Editar historial", in which "edit" is interpreted as an action. Since I didn't know the English version at the time, at first I think the screen for editing the history was not accessible. Actually, there was no history to edit, because in this case "edit" is a noun and this should have been translated, as the help makes clear, as "Historial de ediciones", in which "ediciones" is a plural for "edit" as a noun referred to the various changes the user made on the opened audio file.

If these things happen for popular products in which it is expected that at least a human to review the automatic translations for post-editing if necessary, you can give yourself an idea of how this might difficult moderation and understanding in case of conflict in this forum, where posts do not follow relatively rigid patterns like technical materials largely do. But think on the following cases of non-IT words, in which you can verify the phenomenon through Word Reference dictionaries, which include use examples:

• The English word "cock" as a big variety of meanings, ranging from the male genital organ to various tools for fixing things.[/*

• The Spanish verb "coger" means "to take" in Spain with the sense of grabbing things, but in Latin America is instead used only referred to the sexual intercourse act.

As @14 well said, these cultural and context linguistic hints are not available to machines in the current state-of-the-art. Therefore, for solving human conflicts that sometimes take place in this forum, in case we allow everyone to express in whatever language he or she wants, we would still need to have at least basic notions of the source language which we are asking the machine to translate from.

Also, back into a more technical aspect, how stable the Unicode support of this forum is? I think it is probably outdated, since once I wanted to use an emoji character (the ones that are currently in the Unicode standard, not the old emoticons based on combining certain letters) and the forum gave a strange error whose fixing required I to remove the emoji before sending the post. While it is not a problem for languages such as most of Western European ones, Arabic, Chinese, Japanese, Thai, Turkish, Vietnamese, Icelandic, Greek and the ones with Cyrillic script, many languages are only supported in Unicode (these don't have specific extended-ASCII character sets), so if we want to support posting in multiple languages the Unicode compliance should be accurate.
And even with a stable Unicode support, many screen readers can't speak at all in different scripts (for example a Cyrillic-based language with a Latin script TTS).

I think that in some ways the rule that we can't speak a certain language on this forum is a thing but also not.
It's ok and acceptable to write to your kinsman or to those who you know will understand what you write in that language to do so, and I think a good human being will not or shouldn't stop you, so long as you direct the person with an at sign.
A very good example is when I was talking to Remi in Swedish because I knew he had an understanding of the language, and I know he loves it! Yeah he actually doesn't.
Some people who are hard language people learn how to pick apart a language by hard context, especially if they could use it to make the translater give them the exact output they want.
Funny thing is with a language like Swedish people don't think about it, but it's actually very cloce to English rule, and I mean very dam cloce.
Just one of the words called inte which means not is appended to some words like can followed by inte, and that gives you can not or can't.
But seeing as I use the actual English rule and the fact that I use more than just standard English, I translate inte as aint, as in the word that people from Texas use for am not, is not and are not.
And you know the funny thing? If you write that into a translator it actually works!
Yes it's buggy and problematic, but that's all languages and if the mods and so on want to actually understand things down to the bugs the language has they would have to read up on a lot of English for the ones that are cloce to English and so on, and that's just not easy when they have a big community to help in English alone.
I really love my Japanese people near and far, but your language is too dam wordy although I do love studying it from time to time.
From my name you can tell I'm Spanish and yes I am, but although I love the Spanish people I hate the language and a lot of latin Concatenative languages, especially if they have like 50 forms of the word me or I in the tences.
You telling me that you'll have like 50 words for just the word go and your version of short hand is just scrapping the pronoun and calling it a day? Get the fuck outta here!
Keep in mind that I don't intend to point the stone to anyone all languages have there bugs, just like how I'm still trying to find a programming language that is going to work with my twisted way of processing how raw translation is done.

English is prefered because it's global, it doesn't depend on an entire bunch of consistency to work, it's updatable more than people actually think as in almost every day, and in this regard the huge amount of words helps other people from other country's find a word that they could use to get the message across.

17. I might be the only kurd here. I'm 95% sure

I know Turkish, English and Russian, I have a German which I can call a1 or a2 level, and I know Azerbaijani, too, so I can be considered as a poliglot.
In my opinion, allowing languages other than English to be spoken in this forum would require creation of seperate sub forums and this could cause the moderation team a great loss of time, so even if I wanted this idea to be implemented, I'm sure that it's nearly impossible right now.

While it sounds like a good idea, it just wouldn't work.
The mod team is not very large, and it's already a lot for them to manage the forum in its current state.
Not only that, they'd probably have to get a lot more mods that spoke the different languages that they wanted to make communities for, and a lot of people probably wouldn't want to make such a commitment.
The next issue you'd run into is making sure that everyone is in communication in terms of the rules so you don't have one or more groups deciding to due their own thing and allow pyracy or unethical behaviors.