I've read that Godar or Gestath can teach the skills necessary to get these things from dragons, but the dialogue always leads to something like "There is nothing else I can teach you".

Oh and I found the only other person to have encountered this issue on the internet and it seems that nobody responded to them. Figures.

Sadly there are sometimes bugs in the game for no apparent reason. The only solution I can offer you is to add the skill with the cheat mode. For that open the character status screen and type marvin to activate it and then press either F2 or ^ to open the console and write (can't remember which of the two buttons it was for Gothic 2, so you need to try them):

insert ch

This will spawn the so called character helper which will be able to train you all abilities as far as I remember as long as you have enough points to spent

Believe it or not, I already tried that. With the help of Google Translate, I was able to make sense of the German, but alas, there were no options to learn the relevant dragon trophy skills. Every animal trophy category was blank, except for a "BACK" dialogue choice.

I think I may just insert the armor that I need.

You must be in the right chapter to learn these things. I think, its chapter 4. If you try it in an earlier chapter, you get an answer like: "I cant teach you animore at this time".

Don't worry, I was in Chapter 4. I've moved on from that now. Thank goodness for cheats.