At the moment translations are “plugins” that only let you change the UI language. See the ControlPanel : Plugins : Languages / Sprachen tab. …
The English language is always installed, because it is used as the “fallback language”, is some texts are missing in translations. …
We don’t have a mechanism at the moment, that let’s you have 2 active languages at the same time. …
I do have a draft-pull-request at github that would allow us to create plugins that contain “the same tiddler titles” with different content.
But that’s probably not what you want. … It seems you want to switch the content on a per tiddler basis in edit mode and then “globally” switch language in view-mode … right?
There is some work going on at the moment, that will allow us to be much more flexible with our View- and EditTemplates … BUT that’s not finished yet.
If you could be a bit more specific about your usecase, I think it would be easier to be more specific in my suggestions.