When I visit wikidata.org (with ULS) for the first time (e.g. in a new browser, no jstorage / cookie stuff), it shows me the sidebar etc. in Dutch.
When I click "Help" or something, it then switches to English for me.
Lydia (from wikidata) also had this issue.
We are in Germany so I wonder if it's picking up the caching server ESAMS IP address and using that somehow to guess our language?