Author interface translated

heya,

We're using AEM 6.5 and seeing an odd issue come up, we've translated our sites into language/country folders (eg. "es"->"es-es") for Spanish language in Spain, this is so that we can use MSM. We've upgraded all the way from Cq5 and havent seen the issue before so looking for some help 🙂

Basically, we've got authors who's user preferences are set to "English" using the author, but editing the pages of the "es-es" or "fr-fr" sites and its translating the author interface from English to Spanish/French - how can we set the Author interface to English?!

thanks!