As a native English speaker living by choice in a German-speaking country, I don't agree with their solution.
Personally, I would prefer, and think it would be best, if signs were generally in German. I don't need (or want, really) businesses with cutesy English names. If I wanted to be surrounded by that I'd move back to the homeland.
However, that ship has, sadly, largely sailed. Fine. Whatever.
The actual problem, in fact, is mixing the two languages. It's a horrible habit that people here are learning and practicing far too much. Educated adults may (I hope) be able to do it properly in a pinch, but the youth here are gaining horrible linguistic habits which should not be reinforced by approved commercial usage.
If you want to have a sign using the possessive apostrophe, fine, but make your whole name use English, not just part.
Thus, Evas Brille, or Eva's Glasses.