Where is the formal description of the language that is to be used on Stack Exchange?

I don't think there is one. There is only the Code of Conduct, which does not contain a formal description and there are moderator actions, which are probably based on these guidelines and their personal threshold. I would also say, that a formal description would be too difficult to create as it would have to deal with all the subtle properties of the Language like irony, sarcasm, metaphors, ..

If you are concerned about the way how to say something I would say that writing in a descent, civil manner should be sufficient, i.e. no swearing, no rude or derogatory terms, ...

But that's not everything. What you say matters even more than how you say it. Otherwise "Sorry to see you go" would be a fine, satisfactory response in all case.

Even with the best words, one can inflict harassment or personal attacks. It's kind of difficult to collect all the cases that are not nice and even the Code of Conduct doesn't try that, but the general idea boils down to being nice or something similar.

I guess that moderators who are kind of the guardians of the Language used on the site, have implicitly some kind of internal, probabilistic model of what shouldn't be said. It is probably very complex but it's probably also centered around extreme cases. Most users won't fall in this category.

I always imagine that we are all sitting around a campfire with some drinks in our hands and casually chat about this and that among friends. Of course in real life that's not the case. That's why the common denominator and base of everything is civility, decency and the call to be nice. Most people know by themselves what's right or wrong and mostly they should simply ask themselves: "How do I want to be treated?".

Thankfully, rumored important changes of the Code of Conduct have not yet entered into force or have even been publicized, so I don't need to include them here, but if it happens, I might need to update this answer.

To summarize: If you say something be nice and when you say it, say it in a decent, civil manner. That should guarantee you a high chance of not being booted.

It seems to be custom in this question to end a contribution with a quote. Let me put something showing that language is more than the subject of Codes of Conducts. It's beauty and art and when I was finally able to fully appreciate it in my English lessons, I was blown away.

So long as men can breathe, or eyes can see,
So long lives this, and this gives life to thee.

Where is the formal description of the language that is to be used on Stack Exchange?

I don't think there is one. There is only the Code of Conduct, which does not contain a formal description and there are moderator actions, which are probably based on these guidelines and their personal threshold. I would also say, that a formal description would be too difficult to create as it would have to deal with all the subtle properties of the Language like irony, sarcasm, metaphors, ..

If you are concerned about the way how to say something I would say that writing in a descent, civil manner should be sufficient, i.e. no swearing, no rude or derogatory terms, ...

But that's not everything. What you say matters even more than how you say it. Otherwise "Sorry to see you go" would be a fine, satisfactory response in all cases.

Even with the best words, one can inflict harassment or personal attacks. It's kind of difficult to collect all the cases that are not nice and even the Code of Conduct doesn't try that, but the general idea boils down to being nice or something similar.

I guess that moderators who are kind of the guardians of the Language used on the site, have implicitly some kind of internal, probabilistic model of what shouldn't be said. It is probably very complex but it's probably also centered around extreme cases. Most users won't fall in this category.

I always imagine that we are all sitting around a campfire with some drinks in our hands and casually chat about this and that among friends. Of course in real life that's not the case. That's why the common denominator and base of everything is civility, decency and the call to be nice. Most people know by themselves what's right or wrong and mostly they should simply ask themselves: "How do I want to be treated?".

Thankfully, rumored important changes of the Code of Conduct have not yet entered into force or have even been publicized, so I don't need to include them here, but if it happens, I might need to update this answer.

To summarize: If you say something be nice and when you say it, say it in a decent, civil manner. That should guarantee you a high chance of not being booted.

It seems to be custom in this question to end a contribution with a quote. Let me put something showing that language is more than the subject of Codes of Conducts. It's beauty and art and when I was finally able to fully appreciate it in my English lessons, I was blown away.

So long as men can breathe, or eyes can see,
So long lives this, and this gives life to thee.