"But if you're not using a foreign language, after passing the test you will completely forget it in 2 years" You usually take this test because you will have

"But if you're not using a foreign language, after passing the test you will completely forget it in 2 years" You usually take this test because you will have to speak this language, so the case where you take the test and then do nothing with it seems pretty unlikely. Also, I disagree with your assumption that you will completely forget another language in 2 years (even when not used). I cannot confirm this from my own experience.