Change Language in recovery mode
You can try sudo "/System/Library/CoreServices/Language Chooser.app/Contents/MacOS/Language Chooser"
then choose the system language you want.

Update: I tested it myself.
The above command is only valid for "macOS", it will change the login screen and shutdown message etc.
And the language of "recovery mode" is not associated with "macOS".  

If you want to change the language of "recovery mode".
Just follow the picture,first click File(Dosya) then choose 'Change Language'.