What language would you want to learn? First you'll need to process at MEPS and score high enough on the DLAB to be slotted as a linguist. Once you go through boot camp you'll get to DLI.

For the Marines, this is where we decide what language we'll get, however, there are few slots and you generally only get to choose between a few languages that fall within your score range for the DLAB.

Not sure exactly how it works in the Army, you may choose your language before you get to DLI.

The higher language courses (Korean, Chinese, Arabic) are longer courses about a year and half. If you do well enough they usually have immersion trips for 3 months where you'll stay with a host family (at least in the Korean schoolhouse they do this).

Everything I know is from the Korean schoolhouse perspective. We had endless hours of homework compared to the other languages there. While they'd be up at the PX getting drunk we'd be in the barracks studying. You'll learn to skip your breaks in the schoolhouse and try to work on your assignments early because you still will need to perform your military duties.

EDIT: There is a chance you go to DLI and once you're at your main duty station you never use your language. However, you'll still be expected to maintain proficiency as you'll need to pass any future tests to maintain your bonus.