I am looking for a rather formal verb(phrase) to mean "involuntarily inserting a word from one's mother tongue in a speech made in one's foreign language because the person in question is in a very nerve-wrecking situation such as speaking in court"
A person whose second language is English, and first language is French says the following:
"I don't mean to blow things out proportion, but I think that what you said is, uh, uh, 'injuste', uh, I mean, unfair!" (Heart pounding hard)
Note the involuntary insertion of the French word injuste into the English speech is involuntary because the person was too nervous or self-conscious. The insertion does not come from a lack a competency or command of lexicon, only nervousness.
I thought of stumble but I don't think it serves the purpose well.