fix: translate fallback chapter title

[po5]

As well as handling of (unnamed) for mkv demuxer, empty string for disc demuxer.
https://github.com/mpv-player/mpv/blob/c5211dbf4ae38583b4a55ab63c5c07f8547f73b8/demux/demux_mkv.c#L964
https://github.com/mpv-player/mpv/blob/c5211dbf4ae38583b4a55ab63c5c07f8547f73b8/demux/demux_disc.c#L285
Other demuxers either don't supply a title, or use NULL as a fallback value.

Took care of German and Simplified Chinese too, since those have verified translations on Google.

[po5]

Should we mangle existing chapter titles if they follow the ^Chapter %d+$ format?
It's common for files to have these chapters with useless titles, and it's weird to keep them untranslated.
We wouldn't risk anything bad like accidental renumbering, just match ^Chapter %d+$, override with Chapter %s, and provide string.match(chapter.title, '%d+') as t() param.

Done, cherrypick commits if you disagree.

[christoph-heinrich]

Can confirm that the German translation is correct.

Chapter translations can even be taken a step further by also translating common chapter names like "Intro".
I'm not sure if that's desirable tho.