Let's see how the translation feature works. First, JoomlaPack loads the English files, as a safe default (makes sense, right?). Then, it asks Joomla! "please tell me what's the user's preferred language" and loads those files as well.
Hold on a second. How does Joomla! know what is the user's preferred language? Joomla! defaults to the site's default language, as set in the Extensions -> Languages menu. If a user is logged in, it overrides this setting with the user's preference.
Since the JoomlaPack XML-RPC plugin does not really "log you in" (it simply checks that the username and password belong to an existing user), the language which the plug-in picks up is the site's default language. In your case, I guess it's English.
All right. That's the theory. If your site's default language is Deutsch instead of English (the star icon is besides Deutsch) then, yes, this would be a bug. If not, it's not a bug, it's just a weird feature :D
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!