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.