Feallan said:

OK, Polish subs are displaying correctly in that test file.

Right on, thank you Feallan!

I had a quick look at both the English and Spanish subs, myself (and johnlocke2342 already confirmed that his French subs did NOT display properly in my muxed "test" copy).

The English subs appear to display just fine in the mkvmerge muxed "Sources" test video while the muxed Spanish subs appear to suffer from the SAME issues as the French subs (specifically, question marks appearing in the place of "special characters")…and also similar to the French subs, the Spanish subs appear to display normally when viewed externally in VLC under MacOS 10.6.8 (suggesting that the problem is with the mkvmerge encoding).

I would still like to get verification of the working or non-working status of (both) the muxed German subs from my latest "test" build (available HERE) by either their authors (GeorgeKaplan1959 and deho) or another native speaker, but I expect that I will be submitting a bug report to the developer of mkvmerge shortly.

If I recall correctly, changing the default "Charset" setting to "ISO 8859-1" (as discussed HERE) worked fine for both the French and Spanish tracks previously, so we have a "workaround" for at lest those two language tracks.