So, you noticed your language of choice is missing a string or you just spotted a less-than-ideal translation and want to contribute the updated string, here is some hints for fixing the issue and properly submitting your modifications for inclusion in MantisBT.
Communication about MantisBT localisation can take page at the firstname.lastname@example.org mailing list.
Localization for MantisBT and the Core Plugins is managed thanks to translatewiki.net.
Before you can start contributing, you need to create an account at translatewiki.net
After you have been granted translator rights, you can translate MantisBT Core and the bundled Plugins at the following URLs:
The translatewiki.net staff will add your work to the MantisBT source code periodically (changes are committed on a bi-weekly basis, Mondays and Thursdays), so the only thing you need to do is translate! Some localisation statistics can be found on the MantisBT translatewiki project page.
The same process is valid not only for updating translations for existing languages, but also if you wish to translate a new language. Once the language's translated percentage goes over 40%, it will be added to the MantisBT source code by the translatewiki.net staff.
Run http://localhost/mantisbt/admin/test_langs.php to make sure that the new file doesn't introduce syntax errors.
Note: as the translation files are generated by TranslateWiki, this should not be necessary and is only kept here for historical purposes.
This script is nevertheless useful for validating 3rd-party plugins' language files.