You sure? Because I've only ever seen immortality as 不死. . .

Looked it up, and we're both kinda right. 不死 would be not physically dying. 不滅 is used for concepts (e.g. love). I'd probably actually translate 不死 as immortal and 不滅 as undying in order to keep the same connotations/usage in English as are present in the Asian languages. So, even with Immortal Flames, there's a lot of localization going on behind the scenes, that's for sure. Either way, I'd rather they consistently localize than have them consistently do direct translations.

Note: One of the examples I saw for 不滅 I'm pretty sure was talking about how energy cannot be created or destroyed. So, yeah, not sure immortality is the best translation for that. >_>