As a user, I personally like just having all the strings in a raw text file so I can just throw them in a text editor and spell checker and whatever without going through obscure interfaces and formats and whatever.
As a programmer, the reason I didn't use gettext (or similar) is purely a technical one. All those translation libraries just let you "translate text strings" and that's it, which works fine if you just have a bunch of hardcoded strings that need translating. But once you need to make them modular and moddable that doesn't work anymore, the strings need to have IDs so they can be associated with UI/rulesets, need to be modular to support being split and loaded from different files, etc.
You will notice every moddable game ends up rolling their own format for the same reasons. Yeah maintaining them can be a pain, but nothing that can't be solved with making some support tools down the line.
