In fact with the current state of OPL, you should rather not use any special caracters for game names, only basic latin.
For SMB, the game name is sent in unicode, whereas OPL need UTF8. Unicode share the same basic as UTF8 however, meaning that if you stick to latin is to good.
For HDD, the HDL format use the encoding charset of the system you used to install games. For example, if you use hdl_dump under linux (UTF8), OPL will be happy, and you will be able to display every UTF8 caracters (that is defined in the font loaded). But someone using windows (CP1289), only the basic latin cahrs will be ok (same as unicode/UTF8, there is a partial compatibility)
For USB, I have no idea ... I guess it is tied to FAT32, so probably CP1289 too ...
What mode was it in your case (just to know, as I suppose it did work before) ?
In 0.7 we didn't use UTF8 fonts support, it was rather a fixed charset containing the 256 first chars, probably compatible to latin + latin extended, so some of the special characters of usual countries, including ß, ü, ä, ... That mean it was good for some point, but bad for other ...
And I have no idea to fix this in the future ... we can't support UTF8 for game names, as in UTF8 each character (in fact "glyph") is encoded in either 1 to 4 bytes, and we use fixed buffer size for game names everywehre ... it would require to many changes, and many other problem I have in mind ...