Before I try this program I have two questions:

1) Does it fiddle with my registry?

2) Can it be set to use APE2-tags with Mp3's?

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2) Can it be set to use APE2-tags with Mp3's?

2) Yes, I think so 

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2) Can it be set to use APE2-tags with Mp3's?

2) Yes, I think so

The only way is to temporary rename your MP3s to MPCs, save the APE2-tag and rename them back to MP3s.

Tried this masstagger. Don't find any way to create personal fields (performer, conductor, composition...). Did I miss something ?

I'm trying this nice tagger but the first problem I've found is with the translation... how can I change the language without uninstall and reinstall everything?

I've tried the italian translation but there's too much dialogues still in dutch and this way I can't use the program. Is it possible to leave the untraslated dialogues in english instead of dutch?

- When you try to read-in more than ~ 1500 MPC´s, the program crashes (it totally "disappears").

- The leading "0" (zero) in the number-field is not tagged. For example only "2" instead of "02".

And what I also would appreciate, if there would be a function like in "Tag&Rename" already available:
"Copy IDV1-tag to IDV2-tag" - and the opposite "Copy IDV2-tag to IDV1-tag".

But I've just uploaded a hopefully better and more complete italian translation. With this translation all untranslated strings are in english - not in german.

Simply unpack the file to Mp3tag's program directory

Uhm, Mp3tag does not 'disappear' - every time a unexpected error occours it shows an error dialog and stores some crash information in the file Mp3tagError.log under C:\Documents and Settings\<username>\Application Data\Mp3tag\.

Microsoft Visual C++ Runtime Library
Runtime Error!
Program: c\program files\mp3tag\mp3tag.exe
This application has requested the Runtime to terminate it in an unusual way.
Please contact the application's support team for more information.

Die Anweisung in 0x6c371351 verweist auf Speicher in "0x00000004". Der Vorgang "read" konnte nicht auf dem Speicher durgeführt werden.
Klicken Sie auf "OK", um das Programm zu beenden.

You can enable leading zeros at Options, ID3, ID3v2.

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Uhm, Mp3tag does not 'disappear' - every time a unexpected error occours it shows an error dialog and stores some crash information in the file Mp3tagError.log under C:\Documents and Settings\<username>\Application Data\Mp3tag\.

Well, I just looked into the by you described folder - but couldn´t find any error-logfile.

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You can enable leading zeros at Options, ID3, ID3v2.

I do have to activate then the ID3V2-tagging. But this is not the recommended way MPC´s should be tagged with, ´cause ID3V2 is under suspicion to destroy some of the music-datas of the MPC-files. - Or aren´t the MPC´s tagged with ID3V2-tags anyway, even if this function is activated? - If so, then maybe the program-layout should be redesigned. What about a checkbox besides ID3V1&V2 with APEV2 in this field? - And ID3V1&2-boxes faded grey, as MPC´s don´t need to be tagged with ID3-tags.
Edit:
Just tried tagging with this checkbox set ("leading zeroes"), but unfortunately the result is negative. Still "2" instead of "02".... 

Thanks for the fast debugging. By the way, I yet discovered another slight bug concerning MP3-tagging this time. Another field seems to be wiped-out (cleared), if you tag another one. But I must repeat it to get more clear about it. Later on !

Very cool prog! I hope it will support MPCs with ID3v1.

.... because according to Frank Klemms Musepack documentation it's strongly recommended to store the APE2-tag at the end of the file (and so ID3v1). So a MPC file cannot contain both tags - or am I wrong?

Until now I noticed 2 bugs in Mp3tag:

- When you try to read-in more than ~ 1500 MPC´s, the program crashes (it totally "disappears").