use LANG=C when invoking 'dnf help' and 'sed' with regular expressions

For the completion script to work properly with different languages, you need to specify LANG=C when invoking sed, because depending on current LANG environment variable, regular expressions (ranges like [a-z]) does not work correctly.
It is good idea to use LANG=C when invoking "dnf help" too, because some translations could also break the completion.

This issue is not affected by #181. The completion still does not work with locale cs_CZ.UTF-8, as it is not able to generate the commandlist.