Thank you, I had found this myself in the meantime. This fixes my issues.

That works, but only for the current session. My Management Portal is now in English, but SNMP statuses are still reported in Dutch. The same screen reports:

NLS Locale: NLDW

Can I change this NLS locale to English?

EDIT: I found it . System Administration -> Configuration -> National Language Settings -> Locale Definitions. Select ENUW from the list and click Install. This changes the system locale to English, United States, Unicode.