This article explains why requesting a non-English language may return a mix of English and the requested language in the response.
The response excerpt below is from a request where German was specified. A mix of German and English is returned in the response:
<rateDescription>Double Sea View.</rateDescription>
<roomTypeDescription>Double Sea View.</roomTypeDescription>
This happens when we don't have a translation for the text returned in English. If we don't have data available in the language you requested, we will return it in English, as the data itself is still necessary and cannot be omitted.
You can supplement your own translations for properties you find to be affected, but EPS cannot be responsible for any customer disputes or errors that arise from partner-supplied content translations.