Grammatically, in standard American English, "for shit" isn't allowed. "My English is shitty" is the more standard form. "I can't speak English for shit" sounds like a regional dialect. Then there is the word choice. "Shit" is not a formal word; you should not use it with your boss unless your work culture is very, very, very casual and probably also dominated by young men. If you do hear this word all the time *at your work*, then maybe it's okay for you to use. Don't use it with older family members (or older people in general) or with little kids. For the most part, use it only with your friends.

the s word is commonly used in English to mean something is bad, or not good enough. "my English is like s" "I can't speak English for s" "you can't speak english for s" Etc In the uk we also use the word "Rubbish" in the same way. It is gentler than the s word. Rubbish = the UK word for American trash or garbage. I am not sure if these words are used the same way in other English speaking continents.

Yes, that works. Like Garry says there may be regional or local variation. I could say "My English is shit" or "My English is for shit". Where I am, the 2nd expression emphasizes a little more that it's "shit" :)

Irene has good advice. This is crude language. In my experience, some teenage boys go through a "gangster" stage during which they use crude language. Most seem to grow out of it when they get an adult job. Then, they talk politely and avoid crude, offensive speech.

It's just my opinion, but talking like that usually means you are very young (and trying to get a laugh) or low class. I'm kind of afraid that foreign speakers won't understand when you can get away with this and when it's going to cost you your job.