Here's a curious bit of linguistics: American Students refer to studying languages using a possessive phrase, but not other studies:
a. I have to work on my German.
b. *I have to work on my math.
c. *I have to work on my biology.
Note that all three could easily include the word "skills" at the end, but only (a) is acceptable bare (to me, anyway). I wonder if this is related to the creative work metonymy construction that Pullum just posted about over at LL (it was that post which triggered my thinking on this). Being able to speak a language can be seen as a kind of creative work (i.e., the speaker is producing the language in a way they are not producing biology).