I got his working with the following trick:
Change the resx file with an editor, like this:
<data name="Kundendaten" xml:space="preserve">
<value>Customer data</value>
</data>
Then set the text property of the header from hand to this:
@L["Kundendaten"]
Maybe you can add the possibilty to set the name for tab pages in a future update, that would be a lot easier.