You can change the language of Slideshow’s user interface and web interface either through web interface – menu
Device settings –
Language or through on-screen menu –
Basic settings –
This choice of language has no effect on the language of date/time and weather panel (names of days of week and months can be displayed) – you can select this language separately via web interface, menu
Edit. List of supported languages for date and weather panel depends on your Android image and usually contains 100+ languages on newer versions of Android.