I describe the way my MC messes up her speech because she's not familiar with the language, but I keep her dialogue to English without really including her mistakes. Should I put the errors she makes in dialogue into her actual dialogue, or should I keep it with normal text and simply continue to describe her accent? I worry that adding in her accent would mess up the dialogue and make it hard to read, but I think it would also add a sense of individuality to the MC. Or do I need to add more information on how I describe her accent to this thread for anyone to be able to give me an answer...
I'd suggest including a few grammatical mistakes to indicate her poor command of the language. Don't overdo it, or it might look as if you - the author - are the one with poor language skills; or irritate the hell out of the reader, as he struggles to understand what she says. DON'T write in dialect (dinnae scribe the wa' ye spake) - that's even more irritating to read. But, yes, an example would be easier to comment upon!
Could you write in English with the grammar of her native language? French? No neutral pronoun. Everything is either 'he' or 'she'. German? Leave the verb to the end. Not every single time, just often enough to endear her to your readers.
Anyone ever read "Forrest Gump"? That gave me a migraine within the first 20 pages. I agree; a limited amount of grammar mistakes are a good way to show someone's struggle with a foreign language.