Well, it's actually a crash i've experienced on another mod that uses custom fonts and when used on a gzdoom with the language not set as default english. It was doing the kind of crash where gzdoom exits completely and gives a crash dialog box where you can make a report file and all. So I don't think it's just a vm abort.
When I told the other guy about his mod having the problem, before I noticed the 4.2.0 fixed the issue, he actually found a quick fix. You can eventually add this. He actually removed the SMALLFONT occurence in the fontdefs lump (only that one. He kept the other ones). I also discovered you can just add the lines that can make it crash (the secret found and keys found prompt, maybe more) in the language lump like if you were intending to change them. This had worked for me.
(speaking of the language lump, you did an awesome job with it. It makes teh translation of the mod very easy to do ^^ I wish all the modders works like you)