MantisBT is one of the applications that are localised at translatewiki.net. For software like Mantis there is a limited group of people who benefit. It is therefore very much a "scratch your own itch" that makes people localise Mantis.
This week people started to localise in the Turkish and Vietnamese language. They are making a product their own and as it is open source, every other compatriot can benefit from their effort. Some work needs to be done before these languages feature among the 23 languages that have been fully localised.
The MantisBT website informs about in many ways about its progress; there is a blog and there is twitter. When you are considering your need for a bug tracker, consider Mantis and, you may be in luck with a user interface available in your language.