Is HN forced to be English?

IIRC it's a semi oficial rule. Post in other languages are usually ignored or flagged, unless they are very interesting and there is no equivalent in English.

I'm from Argentina and I can read/understand Spanish, English, Portuguese, Italian, and some German and French. A lot of people here are not native English speakers and each one has a different set of languages that can understand. Bad-Written-English is the common ground.

If you find some interesting article in other language, I recommend to try to find if it has an official English translation and post the translation. If there is no translation, but you find a simplified version in English, post the simplified version and add the main article as a comment and a link to the autotranslation. If there are not equivalents, post the article with a translated title, something like "How to build xyz [German]", post the autotranslation as a comment, and cross your fingers.

If you wrote the article, I recommend to write also a translated version and post the translated version.

Not AFAIK. In fact, there's a 日本語-language article on the front page right now:

https://news.ycombinator.com/item?id=29734021 ("Japan HP accidentally deleted 77TB data in Kyoto U. supercomputing system (kyoto-u.ac.jp)")

If you hang out on /newest you will see non-english stories. (Always spam, in my experience, but I suppose not always) It's hard enough for even good submissions in English to quickly pick up enough votes to make the leap from /newest to the front page. Using a language with a fraction of the speakers would make it almost impossible, I guess.

English is not the native language of 95% of my workplace, but we all use it. Especially with international teams, where there's someone from Iran, Vietnam, China, broken English is the common ground. There's a kind of self-selection that you can't really make it in the tech industry without knowing English, so it's the lingua franca.

You might be interested in the discussion under "Ask HN: Why are all non-English submissions flagged?" at https://news.ycombinator.com/item?id=28086309 .

I think you have answered your own question.