Plugin translation of plugin metadata broken

[DD32]

As the title suggests, A Plugin can specify that its headers be translated via the TextDomain and DomainPath metadata headers, however, this is currently broken.

It appears that $plugin_file was forgotten to be passed to the function, causing the translation to never be loaded.

Patch attached mearly adds $plugin_file as the first arg. to the function, Since this function is designed for private calling only, backwards compatibility shouldn't be affected too much by this change.

[ryan]

(In [11002]) Fix translation of plugin metadata. Props DD32. fixes #9573