The language finder only searches default language files as that's what language plugins and language overrides are overriding. Language plugins tend to be incomplete and missing language keys so it wouldn't be very reliable. Your language plugin should be using the same language keys regardless of the language when translating existing strings. Also note the language finder is just a tool to help find language keys. If you know the key already or want to supply a custom language key you can.

It's extremely discouraging when people want to downplay amazing features and hard work. I've added a feature ticket for it to just search the language keys and strings of the language plugin you're editing.

forge.joomlapolis.com/issues/6432

Language keys don't care what language plugin you're using as language plugins are translating the English strings using the English keys into whatever language, but as the search is also done on language strings I can understand the need for this.

Folks, it's possible to make a feature suggestion without calling something useless.

The search will do partial key searches. Most keys contain the first couple words of a string. At any rate this has been implemented and is under review for merging for next build. It now grabs the language keys and strings then the language plugin keys and strings you're editing and searches them both as needed. In addition to this clicking a language result when there's no empty row will insert one for you and copy the key/text over.