Bug#610910: grub-pc: LC_MESSAGES env value is not used

[Ralf Jung]

Dear grub Maintainers,

four years after being reported (with a patch!), this bug is still open.
I would appreciate if you could kindly review the provided patch, which
looks like a simple enough fix to make grub follow common practice on
Linux wrt. the locale (in particular, LC_MESSAGES overriding LANG for,
well, messages).

Kind regards,
Ralf