Idea

When setting up your profile, you have the option to choose which language(s) that you're learning and how proficient you are in it/them. To users, these options could be vague, or they could not fully understand at which point they are at with the language they are learning. A language proficiency test, integrated into the app, could make these easier to choose for many users, as well as make user selection more accurate

Benefits

Users would be able to more accurately select their skill level in a language, as well as remove confusion about what skill level they are at.

Implementation

Due to the time it may take to complete a test, you can make it optional, however for the users who do complete the test, they could have a marker next to the language that they have taken the proficiency test in, to show users that it is an accurate representation of their skill level.