The passwordstore lookup plugin depends on parsing GnuPG's
error messages in English language. As a result, detection of
a specific error failes when users set a different locale.
This change corrects this by setting the `LANGUAGE` environment
variable to `C` when invoking `pass`, as this only affects
gettext translations.
See
https://www.gnu.org/software/gettext/manual/html_node/The-LANGUAGE-variable.html
