Pay will vary by:
1. The language you speak (I believe they stopped paying Spanish speakers).
2. Your proficiency
3. The requirement for you to use the language in your current position.
Go down to your post Education Center to sign up for the test. See how you do. They will help you through the process. You might also try the Military Intelligence (MICO) unit on post. They are typically familiar with the current need in each language.