I'd try to keep it simple and go for something like "beginner", "intermediate", "advanced", "native level". In your case, "(mostly) passive knowledge" may be imprecise but would probably be enough, as details could be given individually in the profile description.

I think it most important that members indicate their native language in their profiles and that non-native contributions are automatically marked as needing verification.

Edit: I'd like to add that I hope a way for users to indicate their native (or strongest) language in their profiles will be implemented in a future not too far away.

Maybe we could recommend some on-line test for those who can't precisely estimate their level, for example,
http://www.testden.com/challeng...FeHVcgodZTsA3A

I believe there are similar links for other languages, too.

I'm partial towards the ILR scale, compared to CEFR: https://en.wikipedia.org/wiki/ILR_scale

ILR is simpler and has less levels (more levels = more vagueness and incorrect assessment).

Perhaps we could make our own system. These scales are generally made with dedicated speakers of languages. On Tatoeba, however, many of us are language nerds and therefore, polyglots to varying degrees.

Going off your Vietnamese example, we could use a system were a language is listed on a user's profile, along with a table of icons representing writing, speaking, reading and listening, which are then colour coded to represent level: grey for no ability, red for poor ability, orange for medium, green for good. Native languages are represented with a special gilded row, a star or a similar mark.

So mine would look like:

Marathi (gold) : [L: Green] [R: Yellow] [S: Green] [W: Yellow]
English: [L: Green] [R: Green] [S: Green] [W: Green]
Hindi: [L: Green] [R: Yellow] [S: Green] [W: Yellow]
Urdu: [L: Yellow] [R: Red] [S: Yellow] [W: Red]
Japanese: [L: Red] [R: Red] [S: Red] [W: greyed]

If it's too complicated, we could leave this as a 'fine-tune' option and allow readers to pick based on a simpler colour coded system based on ILR: 0: Grey, 1: Red, 2: Orange, 3: Yellow, 4: Green, 5: Gold (picking one of these automatically fills the rest of the LRSW icons with that colour).

> we will need to add a feature for members to indicate what languages they speak and how well they speak it. But what kind of information will we need exactly?

To answer this question, we need to ask ourselves what do we need this information for. If we need to know how well someone masters a language so that his or her contributions should be classified as trusted or not (for instance automatically tagged as NNC), two levels are required: native or not.

If we consider language skill as a way to tell users in what languages one is able to communicate with them, then I’d add a third level: able to read messages from other users.

Of course users are free to add more details about their knowledge in their profile description, like your oral understanding of Vietnamese, but I see no need to store more details in specific form areas if they are not gonna be used for your initial objective (improving the quality of the sentences).

>we will need to add a feature for members to indicate what languages they speak and how well they speak it.

Avez-vous prévu un détecteur de mensonges ? Parce qu'il y a quelques menteurs ici, même s'ils ont parfois réussi à faire accroire qu'ils étaient natifs et à se faire estampiller comme tels par CK...

Et avez-vous prévu la détection de mythomanes ?
La plupart des gens surestiment leurs niveaux de langues.