Convert export Form to Drupal 10.

There is a tiny bit off topic change which is about the importTitle() and import() methods not leveraging the $group argument available at their disposal. Since i was changing it for the export() methods, i also updated the two extra lines.

There was an incorrect asumption in the actual code, the \Drupal\l10n_community\L10nExporter::export() method doesn't work if the language is undefined so i removed the nullability of the argument.

Since those methods are only used in the export form it felt ok to do it in this MR.