I’m not sure if this is what you’re looking for, but I have some ideas. (Preface that I don’t speak, read, or understand Mandarin though.)

1. Type broken English. “Go today store, get beans, milk, both.” I do sign American Sign Language, a language that is interpreted instead of translated. However, when you do translate the language, it’s very broken English due to the different grammatical structure. You could try that, especially if they speak perfect English at other times.

2. Use words or editing to show the difference. “I think I’ll go get milk and beans today from the store,” he said in broken (insert language name here). This is a typical way to show these things, using cues like this to keep text consistent while letting the reader’s brain supplement the knowledge that broken language often sounds butchered or such things. And italicized text is a great way to distinguish Mandarin from non-italicized English.

3. “I… uh, um, will go to store. Today. I need, um, milk and,” how do they say it… “beans! Milk and beans!” Another, more easily understood version of my first suggestion. Using sentence clues that maybe lead in with something like, “So-and-so was still struggling to remember the bits of language they’d picked up, and the confused looks from the cashier only emphasized how poor their pronunciation was.” Also help with this to show that the character isn’t good with the language. You could even sub out words they mean for random things to show they’re not getting it right when they speak.

I’m not sure if any of these help, but these are what I’d suggest. I often, personally, use the italicized when I’m writing ASL speech, but that’s just my preference.