First up.... I am fluent in speaking and writing in several languages. I can get along in several others in the same way. Others on top of that I can get along in just conversation or muddle through. I'm no genius but languages build on others and it depends on life experience.
Now sometimes you can stipulate in your contract a specific language. But you have to show that you studied it already or have had exposure to it through family. Or have enough other languages under your belt so they realize you are a good bet to learn another.
Even then it is the needs of the military that count. There are many people who studied a romance language in high school, college or been stationed there (European languages) and once you know one you can generally pick up the others. If you speak English it is not so hard to learn them regardless. But Europe is not the focus now. Of course there is always a need but its not what is going to really get you there.
Of course there is a shortage for Arabic, Farsi, Pashtu speakers and whatnot. Of course you would pick up or be sent to school again for the different dialects.
Look up AFRICOM. America is forming a new command. English is the primary language of diplomacy and business worldwide, Arabic languages are key and French still holds sway there. But obviously languages that are still used (like Swahili) are going to get you somewhere.
You have to think about what you might do well in, what knid of job you want and how marketable your language will be if and when you get out. You have to look at politics and trends. In picking a service ask how likely it will be that you would get assigned there. If you joined the Marines your options could be limited as opposed to any of the larger services. Then you have to factor in quality of life or if you plan on having a family.
Its complicated but not.