To see why David Sedaris sprinkles scrambled nonsense words like "meimslxsp" and "lgpdmurct" into his essay titled "Me Talk Pretty One day," rather than real French words, we must have a clear understanding of his purpose and thesis. His purpose is to describe himself as a 41-year-old who has just returned to school to study the French language in Paris. His thesis can be found farther down in about the third paragraph in which he describes his first day of class as a "nerve-racking" experience. More specifically, he asserts his surprise to find that, despite not considering himself a complete novice because of all the summers he spent in Normandy and having taken a "monthlong French class" in New York, he was only able to understand half of what his Paris French teacher was saying. After stating this all important thesis that he only understood half of what his French teacher was saying, he illustrates it by quoting his French teacher in such a way that he translates into English what he was capable of understanding in French and using gibberish to quote what he couldn't understand, as we see when he says:
If you have not meimslsxp of lgpdmurct by this time, then you should not be in this room. Has everyone apzkiubjow? Everyone? Good, we shall begin.
From this we can understand that every single word he correctly said in English were the words he was able to understand; the gibberish words were French words the teacher said that he couldn't understand at all. Since the words sounded like gibberish to the author, he wrote them as gibberish for us to see simply because that's the farthest he could get with the words. The approach is extremely effective and comical because the reader is able to relate to not being able to understand certain words being spoken out loud, especially if the reader has studied a foreign language or been in a foreign country, plus able to clearly see the poor author sitting in the classroom feeling befuddled and frustrated. In short, Sedaris included gibberish words in his essay in lieu of real French words to indicate he did not know or recognize the French word he was hearing.
Sedaris made use of gibberish words in "Me Talk Pretty One Day" to illustrate how he felt in his attempt to learn the French language while in France. The fact that his French teacher often mocked him and his inability to properly pronounce certain French words offers an example of how embarrassed and frustrated the author felt when he felt as though he heard gibberish in his class and on the streets, and he felt as though he only spoke in gibberish to the judgmental people around him. This more than likely hearkened back to the author's essay "Go Carolina", in which Sedaris conveys his feelings as a young boy who was forced to see a speech therapist in order to rid him of his lisp. This tells us that Sedaris has always had an issue with being understood properly and has throughout his life been made fun of for his speech. Using gibberish words to illustrate this fact in "Me Talk Pretty One Day" was Sedaris's way of using humor to covey his feelings of never being understood.
The answer above is wonderful in describing why he used gibberish to show the words. Instead of saying the French word, he was trying to show more of how exactly he felt. I had a French teacher who liked to throw out a lot of random French into his sentences and if it was a word I didn't previously know (because he would say words he had not taught us) or his accent was too thick (which was often) then all I would hear was the same gibberish. He most likely put in the spelling like that not only to make the reader feel like they could relate and also to bring some humor to the essay, but he also may have genuinely not gotten the word. He could know the words if the teacher said it now, however it can be difficult to remember the exact words used if you don't know what was said from the start. He genuinely may not even know which French words were used when he was doing his quotes and/or learning from the teacher. This means that he could guess the correct words to put in, but may not remember the real word at all because it didn't sound like much of anything at the time. It adds an even higher relating aspect to his essay.