You can - respectfully - ask. It's that simple. "Please, it's difficult for me to follow conversations that switch languages frequently. Would you mind keeping conversations in English around me?" There's a decent chance the answer will be "sure, sorry to confuse."
Of course, the next question is "how do I get them to follow my ...
Imagining you are a passionate sandwich maker and your colleague approached you in a similar fashion:
What if instead of one you made yourself a couple of sandwiches? Wouldn't it be awesome ?
And you actually know he's planning to eat the remaining sandwich.
You explain that you tried the following responses:
Yea... you're right... do a single ...
If I understand correctly, this person repeatedly says things to you in this form:
You should do Action, because Reasons.
The actions and reasons vary, but you feel that consistently, Reasons are either false or are leaving out C's actual reasons for suggesting this action to you (or wanting you to take this action), hiding them behind reasons designed to ...
You need your own formula.
The good news is that your Buddy has no power over you, other than the power you give away.
Prioritize what you can control — your response to a predictable scene.
Ignore what you cannot control — insecure Buddy with a repetition compulsion always at the ready to manipulate.
All you have to do in handling this perennial ...