earwig
How to localize this tool?
There is no system in place for translations, mainly because no one's asked for it and I can't do it by myself (I only speak English). But I'm glad you've brought it up, because it's a good idea.
I don't have a ton of free time right now, but it's something I'm willing to work on. Do you know what the standard methods are for translating Wikimedia-based tools now-a-days? I know translatewiki.net exists, should I try to get copyvios on there or is it not worth the trouble? I know babel exists, which may be useful.
For this purpose (localization of tools in wmflabs) was developed especially framework - see https://tools.wmflabs.org/intuition/
The translations themselves can be made in translatewiki.net
(some context for my request: Your tool was recently added in the Russian Wikipedia as regular external tool in each history page)
Intuition looks to be PHP-only, so I don't think I can use it. But translatewiki should work for the actual translations. I'll look into setting this up soon.
And I didn't know about the Russian Wikipedia linking to my tool. That's pretty cool. It explains why we've been getting more hits than usual!
About translatewiki - see https://translatewiki.net/wiki/Setup_of_a_new_project