[girigiri]

Good idea, but it doesn't have to be so rough on the moderators.
An easy and reasonably effective method would be to simply add details in the profile section.

My command of Japanese

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0 Do not know any Japanese
1
2 (know hiragana, katakana, some basic phrases)
3
4
5
6
7 native speaker / JLPT 2 equivalent
8
9 excellent

Self rating works reasonably well, and dishonest replies can be dealt with in the same way as spamming.

As to a sticky logo or whatever - hmmm ... Maybe it is something that the avatar makers here might like to play with. If it comes down to it, I'd be happy to host suitable pics.