Part of being a Foreign Service Officer is language ability and there's no way around it. Language skills are a must and you must be prepared to learn new languages if you don't know any already. While knowledge of a foreign language is not mandatory to become an officer, but it is helpful. Knowing a political language (what I define as a language with political currency at a international level), can net you between .17 and .38 points for the Register. The bonus may sound puny but this is pretty good.
If and when you pass the Oral Assessment, you are given a performance score out of 7. A minimum passing score is 5.3. You are then placed upon the Register, a list of candidates for A-100 within your cone. The Register is, in essence, synonymous with a performance rank. The higher the score you received during the OA, the higher your rank. Redditor TheSouthMouth got a 5.6 on the OA, which was sufficient for him to be ranked #15 of 94 in the Economic cone. He ranks among the top 20% on the Register, which is a very strong spot. Blogger Valdysses got a 5.3, which earned him #100 of 140 in the Public Diplomacy cone. So as you can see, every decimal point counts, and the bonus received from the language test can make the difference between 15 and 100.
Most languages listed by State require a level 3 speaking ability and will earn you .17 points. To see what that is like, watch this.
Eight critical languages can be taken as well. A level 3 speaking ability in one of them will give you .38 points. A level 2 will give you .25 points. To see what a level 2 ability is like, watch this.
By the way, after Valdysses languished at #100 for more than a year. After he took and passed the Turkish language test, his rank went from #100 to #12 of 181 and he was hired in two weeks.