In which way is the language choosen?

We created a BSP-Applikation some time ago and it's on customer site for some years now. A customer is wondering about the language selection in his system.

If a user logs on the Applikation in a language where no translation exists he is getting the OTR-texts in German. He expects to get the texts in English. In his meaning the issue is caused by the fact that the OTR-texts have their original language in German. I don't think that's causing the issue.

His system has standard language English so I would assume that the texts are displayed in this language if they are not translated.

Does anyone know how the automated language selection in BSP works when the choosen language is not installed?