I'm searching for slang or informal words that describe person who is bad at language, especially English. Is there anything like this in British English or American? I think about people who had moved to - for example - UK, have lived there for years, but their language skills are still low and full of grammar mistakes. I would be really thankful if you could answer.
Sadly, in English we lack anything as colourful as "parler français comme une vache espagnole" (to speak French like a Spanish cow) although a similar phrase "He speaks English like a native" can be used either with obvious sarcasm or with a delayed or implied "...of Outer Mongolia" or other faraway place.
The other one you may hear is that someone is 'mangling' the language - from the OED:
To make (words) almost unrecognizable by mispronunciation; to spoil, corrupt, misrepresent, or do violence to (a text or extract, or the meaning or essence of something); to distort (music) in a similar fashion.