@whitesilly
I cannot replicate your issue. I have set German as the Primary Language and added English as a Secondary Language. I then updated the Sign In page template with German and English translations.
German

English

I then visited the Client Portal, selected German as the language, and clicked Sign In. I see the German Translation I added above for both German and English.
German

English

Please provide us with EXACT steps to replicate this issue. How are you saving the content, etc.?
Cheers.