For the language files, there is a need to have two encodings for the same language. The conversion (from ansi to utf without bom) can easily be done with eg Notepad++. This is needed for all language definitions and templates.
There is no need to rename the language directories itself as Trabis suggests, but the archives should contain two extra directories in the root: iso and utf, which will then contain the normal directory tree with all the language files.
A partial example for the mrbs module and french could be:
|_modules
| |_mrbs
| |_index.php
| |_xoops_version.php
| |_lang.fr
| |_admin
| |_language
| |_french
| |_main.php
|
|_iso
| |_modules
| |_mrbs
| |_lang.fr
| |_language
| |_french
| |_main.php
|_utf
|_modules
|_mrbs
|_lang.fr
|_language
|_french
|_main.php