Add language filtering in needed place

[Shadowner]

I propose adding language filtering to the following boards:

• /admin/ (users board) (matching the design of the TrollBoard, to add other search field like name, mail and other in the future)

• /dashboard (top 5)

• /leaderboard (matching the design of the TrollBoard)

I would love to work on this development, but I need to wait for PR #2072 to be reviewed and pushed, as it adds a component on the website side that could facilitate the development."

You can assign me to this and I will do it when the PR is reviewed and this issue reviewed to ☺