The army and marines, who are actually most in need of linguists, have not yet finished working out the details of who they will pay, for what language and how much. However, it's expected that the army and marine rules won't be radically different from those used by the navy and air force.
The Department of Defense has also created language training programs for needed languages. Troops can learn via websites ("remote learning") or attend classes in some bases or cooperating outside schools.
The U.S. Department of Defense effort, to encourage more troops to learn foreign languages, includes larger monthly bonuses for those who can speak and read needed languages. Congress authorized the money, and the larger bonuses, but left it up to each of the services to handle the details. The U.S. Navy and Air Force have now come up with a largely identical set of such details. Bonuses range from $200 to $500 a month, depending on the language. All the languages of the world are divided into three categories of military need; "immediate investment," "strategic stronghold," and "other" (which gets the smallest, or no, bonuses.) The "immediate investment" languages include the usual suspects (Arabic, Chinese, Iranian, etc), and if you know two of these, you can get up to $1,000 extra each month (the highest "multi-language" bonus.) In addition, there are a number of languages, that used to earn a monthly bonus, but no longer do because language skills for them are now, "abundant or surplus." This list includes; Spanish, Tagalog (Filipino), Portuguese, German, Italian, Russian, Korean and French. You can still earn low level bonuses for these languages if you need to speak them in order to do your job.