I am a 15 year old native English speaker with some knowledge in Latin. Which language could I learn that would look best on an application? I am willing to put the hard work in to learn the language and i was thinking of doing Russian.
You sign up for the language that really interests you, full stop.
If you want to be strategic about it, then think about broadening your desire to serve. What's your plan B, C....Z if you do not get into USxA? What's your plan B, C....Z if, for some unforeseen reason, you are DQ'ed from commissioning into one of the armed services? Would you then enjoy and pursue one of the ABC agencies, as
@Capt MJ is fond of asking (wisely)? If so, a language like Russian would place you well -- if you like it enough to do well in it. It's also pretty clear that Russian, Mandarin, Farsi, Pashto, Arabic, and variants thereof will be strategically important in the coming decades.
All that said: the most important part is to take a language, any language, and do well in it. When you learn your first non-native language, you realize things like how idiomatic English (American E., British E.) is (are), and how hard English is to learn as a second or third language. If you learn Russian, as I did in college, you realize what a PITA it is to learn the six verb cases. However, those six verb cases make a statement like "I am going to the library" по русски very, very precise: you will know whether the speaker is going immediately or sometime in the future; whether they are walking or driving; whether they intend to make a return trip right away or after a while; whether they are going with another person or people. All from one verb choice! That insight, even a peek, at how other languages work can be fascinating and really important. It also helps you learn other, related languages later. If you become a language Jedi, you can learn to use language in cyber and espionage applications. So, learn something -- anything -- try it on for size, experiment, play with it even as you grapple with it. Then go from there.