How do I force prompts to be in English

I installed WinSCP in German.

When I use WinSCP.com for scripting I get promts in German (e. g. "Aktive Sitzung"). How can i force WinSCP.com to prompt in English although the user interface for WinSCP is set to German?

First, you should isolate WinSCP script run from GUI configuration:
https://winscp.net/eng/docs/scripting#configuration

If WinSCP defaults to German anyway (because you have German locale set in Windows), force English by:

winscp.com /rawconfig Interface\LocaleSafe=1033

https://winscp.net/eng/docs/rawconfig