The language of the website can be changed in the right upper corner with the little downward arrow. Once you've selected your language, all programmes available in the chosen language will appear. Same for the app: Tap on the little gear icon in the right upper corner of your ARTE app and choose the language you wish.
You can also change the language on a programme directly, by clicking on the little speech bubble icon (or gear icon in the app) at the bottom-right of each video. On smart TV, you can change the language at the bottom of the page.
Unfortunately, due to distribution rights restrictions, not all programmes are available in all language versions and in all territories.