Just signed up for this but having trouble adding cards.

I only have German cards, but searching for those seems impossible, nothing is being found.
Do I really have to first translate the cards into English, then search for them?
Also, if I have found the English card and switch its language to German, nothing changes (apart from the flag icon). The card is still listed with its English title in my inventory.

Am I missing something or is this not possible? If not, then this service is not of use for me :(

It is sadly not yet possible. It's on our roadmap but did not yet have the time/resources for internationalization.