fixing issues with UTF8 characters in translatable strings

Currently the l10n script to extract translatable string assumed ASCII as character set and failed e.g. in firstrunwizard:
````
    Reading ./lib/firstrunwizard.php
    Reading ./templates/wizard.php
xgettext: Non-ASCII string at ./templates/wizard.php:34.
          Please specify the source encoding through --from-code.
    Reading ./wizard.php
````

In order to fix this the character set is set to utf8 on the xgettext call.

Patch has already been applied to the apps repo.

@karlitschek @jakobsack @blizzz THX