passwordstore: Fix error detection for non-English locales (#4219)

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

plugins/lookup/passwordstore.py

@@ -229,6 +229,7 @@ class LookupModule(LookupBase):
 
             # Collect pass environment variables from the plugin's parameters.
             self.env = os.environ.copy()
+            self.env['LANGUAGE'] = 'C'  # make sure to get errors in English as required by check_output2
 
             # Set PASSWORD_STORE_DIR
             if os.path.isdir(self.paramvals['directory']):