This repo hosts the flatpak version of Opera GX Browser
You can change the browser language by adding a startup parameter lang, full example:
/usr/bin/flatpak run --branch=stable --arch=x86_64 --command=opera --file-forwarding com.opera.Opera @@U --lang=cs %U @@
You don't need to change anything to get video working, this flatpak automatically includes FFmpeg prebuilt codecs in the location where Opera GX expects them.