I think, I looked at the wrong spot... By coincidence I found the "Settings Tab" following "Compile Questionnaire" and heyjeyyy... there it was (and already chosen) - additionally to a more than comfortable template to enter all different language versions.
BUT: I had to implement the translations not - as I planned it as direct input html on page 1, but as stand alone "question / text" on theis first page.
This even seems now logical to me.
Regards for your help,