what language would be best to take at West Point and for use outside and inside the army.
This is a good question to ponder, but the wrong question to ask at this point.
Russian, Arabic and Chinese, you know...
This would be the right answer with the possible addition of Farsi (Persian). These are three of the most difficult languages for English speakers to learn. It is a long road between choosing and language to learn and becoming proficient enough to be of any relevance to your career.
Do you have any demonstrated aptitude for learning a Foreign Language? Do you have a strong interest in the history, culture, or geopolitical political importance of any of the countries where a particular language is spoken? Unless you are blessed with a gift, the answer should yes to either or both of those questions. Spanish or German would be considered the easiest languages, but even then, many smart people struggle with them.
The Army has a general interest in promoting foreign language ability of its officers, but it will be of little or no relevance to your service until the rank of Captain...and that assumes you join Special Forces. Even then, you could have mastered one language only to be assigned another one altogether.
That being said, Americans generally don't value proficiency in a foreign language as necessary or even important. Nonetheless, it is an excellent addition to one's skill set. It can be called upon, sometimes inadvertently, and allow you to differentiate yourself from your cohort.
Congratulations on your appointment and best of luck!