after some tests I reached the same conclusions. Sitefinity has no localized labels out of the box, so you probably imported Dutch language some time ago and now when switching the frontend language, the backend language changes too, with a mixture of translated labels and ones in English.
The solution is rather easy - export the English language pack from your instance of Sitefinity and import it as Dutch. This should overwrite all translated backend labels.
Daniel, feel free to get back to me for more instructions.
Grisha 'Greg' Karanikolov
the Telerik team
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