Metonymy: a figure of speech in which a thing is not called by its own name, but by the name of something intimately associated. For instance, "Houston" for NASA Mission Control.
Sociolect: a.k.a. a social dialect, a variety of language associated with a social group such as a socioeconomic class, an ethnic group, or an age group.
Cline: in linguistics, a scale of continual gradation.
"I speak English. I'm learning Language X." These two sentences together carry a fallacy. The fallacy is that we fully know our mother tongues ("tongue", by the way, is an example of metonymy, but I didn't know that an hour ago).