Exccellent!
Two things:
I think not at the moment. The problem is that no application supports that (except of Frontah to my knowledge). I know that someone has to make the first step to support this, but I'm not sure if Mp3tag will be the first - we'll see 
The problem with enabling ISO-8859-1 by default is, that characters which are not part of this character set will not be written to the file. dano suggested to test this internally and write UTF-16 tags if the conversion won't be lossless, but I still had no time to implement this.
Anyway, it would be great if users who have problems with other apps, ask their developers to support ID3v2.4 with UTF-8 too.
Thank you
Yes, VERY good job, big thx from me too 
As for appended v2.4 Tags - I don't like these, no one supports them
and there's a better alternative (APEv2). So no hurry needed
in that direction 
I see where you come from, since 2.4 doesn't have much support. And yeah, APE are a "better" alternative when it's supported. id3 is just a huge mess.
But I believe appended tags are better for quick tag editing without the need of padding or risk of corrupting the file. I prefer all my tags appended just like all other good tags that are placed at the end like APE, and the original id3v1. Placing id3 tags at the beggining is just time consuming and a pain to edit in large quantities.
Yes, ID3v2 is hard to implement, bit when it's done properly (as in mp3tag or foobar2000 or in all programs using the TagLib Library) I see no big problem.
Support for ID3v2.4 is increasing (see Winamp), the same cannot be said for
APEv2. And it's more "native" or suited for mp3s since it was designed for mp3s.
I recently got to know that APEs might cause more serious issues for mp3 decoders,
whereas the worst which usually happens with ID3v2 is that the tag is not just read.
APEv2 (sadly) was more like a makeshift solution, which never got player support
outside foobar2000.
Appended tags are certainly better, but its so uncommon for v2 tags to be appended
practically nobody supports it, and I think it's unlikely that support for that might grow...
whereas support for the common prepended v2.4 tag type might finally have a real change to catch on in the long run. And this might be as good as it will ever get for mp3s 
Cheers!
Klaas
XMPlay also supports APEv2 tags. 
This is something like a chicken-egg problem. Software developers are not implementing it because it's not widely used - it's not widely used because there is no software.
While I don't really need padded ID3v2.4 tags, it wouldn't be bad to have them (not now, but in a future version of MP3Tag when Florian has more time). ATM, I am more than happy with what MP3Tag can do and it's by far the best tagger I ever used (and its evolution over the years is amazing, too).
OK 
Hehe, that's exactly what it is. What I wanted to make clear is that this
has to be the hardest "part" of ID3v2.4 to get support for in the future.
Amen. MP3tags feature set at the moment is just 
As Sebastian Mares, I was also aware of XMPlay supporting tags. In fact, it supporting APE was the reason I switched to XMPlay's great audio player (imho the best), and started tagging away everything in APE.
But as soon as I learned programs were finally giving appended ID3V2 tags its merit I'm making the final switch to ID3. And, as klassklever mentioned, it's the intended tag for MP3s.
Don't we all like Mp3tag for the same reasons? It's innovative, functional, incredibly simple to use, and always at the top of compatibility. And it keeps evolving to be always better than the rest. No wonder it's the best tagger I've ever seen.
Btw I will have to loudly disagree. XMplay is hands down the best one. I mean, which player have you seen that can read all tags existing in a file? Certainly not Amarok.
And like Mp3tag, it should be released for Linux. Can I dream of this?
Florian has it in the right direction. 
I'm very very glad to know that my request, has turned into an mp3tag feature.
Thanks Florian.