Output boolean attribute changes in language

[htmagedev]

Hi, I have this very weird problem: A datafeed is generated by cron. It writes product attributes to a XML file. Boolean product attributes (yes/no attributes) are written as text (default magento behaviour: Yes/No). However, every time the feed is generated (every 15 minutes) the language changes from English to Dutch and back. So in the feed I see the same attribute written as "Yes" and "Ja" or "No" and "Nee" and is swapped (Yes<->Ja and No<->Nee) with every run. The Dutch language is used in the backend and also in both (and only) store views.
This behaviour started to notice when I changed the value for "Schedule Ahead for" to 60 (was 20).
Any ideas?
Settings, see screenshots (attachments).