A request for a word similar to 'stumble in speech'

Question

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"

Example:

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.

Answer 1

I can only think of "reverting to one's mother tongue". http://www.answerbag.com/q_view/312196

Answer 2

The formal term is code-switching, and the specific sub-category is tag-switching for switching words or phrases. It happens involuntarily also.

Tag-switching is the switching of either a tag phrase or a word, or both, from one language to another, (common in intra-sentential switches). In Spanish-English switching one could say, "Él es de México y así los criaron a ellos, you know." ("He's from Mexico, and they raise them like that, you know.")
- Code-switching/Wikipedia

Answer 3

I would use Momentary Lapse.

Lapse on its own would also be appropriate but could imply that the speaker inserted a larger section of speech from their native language.