There are many funny videos of people imitating speech in various languages by only focusing on the sound of the language rather than content. The speech is usually completely invalid in given language but sounds valid to someone unfamiliar with the language. Is there a word for such kind of imitation?
Mock language or more specific, like 'mock French' or 'mock German'. Note that it is sometimes considered racist to use mock language (try googling mock Spanish, you'll see some discussions of this.)
In this link the term 'mock language' (quotation marks are used on first mention) is used:
Abstract: This study explores ‘mock language’ by examining borrowed words in Canadian newspaper data. Mock language refers to the (usually negative) connotations that emerge as a result of the dual indexicality of borrowed words. Dual indexicality functions by allowing speakers to draw on stereotypes about other languages and speakers, thus reinforcing the role of languages as boundary marking devices
Glossolalia or (speaking in tongues) is the fluid vocalizing of speech-like syllables that lack any readily comprehended meaning.
From the Skeptic's Dictionary, Glossolalia is fabricated, meaningless speech.
According to Dr. William T. Samarin, professor of anthropology and linguistics at the University of Toronto, glossolalia consists of strings of meaningless syllables made up of sounds taken from those familiar to the speaker and put together more or less haphazardly .... Glossolalia is language-like because the speaker unconsciously wants it to be language-like. Yet in spite of superficial similarities, glossolalia fundamentally is not language.
How about mimic? It's not a perfect fit but describes the action fairly accurately.
Imitate also works well. http://dictionary.reference.com/browse/imitate?s=t. The 4th definition is commonly understood.