clipmediaprops: Handle potentially broken framerates such as 1000 fps

Fixes bug #721888
check: clarify Libav as GStreamer Libav
i18n: clarify Libav as GStreamer Libav

This change updates the "formal names" in i18n files
to match the message change in check.py.
Show an error dialog when encountering Unicode decoding errors/broken locales

Fixes bug #729523