Where is the code which defines the language (correct .po file) Membership plugin?

Hello,

So it appears that I have some issue with localizing the Membership plugin. What do I need to edit to change the localization from the default
membership.po
to another .po file. Or cuz I noticed that the .mo file had the us_US designation the same question applies.
I checked the includes and I cannot seem to find the language variable.

I went ahead to try and achieve this with the Codestyling Localization plugin however: "Warning: The actual loaded translation content contains mixed textdomains and is not pure translateable within one textdomain." which as I can see is due to a few includes - nevertheless I am unable to translate the plugin.

Usefull insight will be greatly appreciated. Thanx