Once the episode starts, click the "Audio & Subtitles" icon (top right corner). Select English [Audio Description] or English for audio, and French (Français) for the subtitles to achieve the "VOSTFR" experience.
If you are looking at file listings or community guides, here is what those specific technical tags mean: Season 3, Episode 1 ("The Apostate"). The Mandalorian S03E01 VOSTFR HDTV
"Version Originale Sous-Titrée en FRançais." This means the original English audio is preserved, and French subtitles are hardcoded or included as a track. Once the episode starts, click the "Audio &