Menu / Toolbox / Preferences / Preferred Subtitle Language
The above will pick the subtitle language you’d prefer, assuming that the broadcast contains more than one subtitle language and one of them is the one you select. This is unlikely.
While watching a programme: ok (or enter) / Audio & Video / Subtitles
The above will list the subtitle languages that are being broadcast at the time. It’s likely to be just the one, and in the native language of whatever’s being shown.
So … ARTE contains programmes in German and French. Some will be French dubbed in German (with French subtitles) and some will be German dubbed into French (with German subtitles). There may be some that don’t follow this pattern, but most will.