Few strings not localised yet
Here are few strings missing localisation. (There are not in catkeys file so can't be translated.) In about window, "All rights reserved" & "Special Thanks:" In main window, in bottom left corner "Text File" and I guess whatever filetype is.
That's all I found.
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The strings you mention in About window are from Haiku: https://github.com/haiku/haiku/blob/d87eeceea5d64289678d5796bf528b7844937ac0/src/kits/interface/AboutWindow.cpp#L502 https://github.com/haiku/haiku/blob/d87eeceea5d64289678d5796bf528b7844937ac0/src/kits/interface/AboutWindow.cpp#L530
File types are not translated yet, indeed. They are loaded from YAML files, so making them localizable needs a bit more thought.