SNES version-
http://www.gamefaqs.com/console/snes/game/588331.html (English/Spanish/Portuguese/Dutch...no German)
http://www.gamefaqs.com/portable/gbadvance/game/930369.html (only English, no German either...)
There's some guides there but unfortunately there are none in German. Only English/Spanish/Portuguese/Dutch.
Basically there isn't THAT much you can miss. In World 1, make sure you get Shiva from a secret passage in Worus Castle. There's also something hidded in the basement of the castle near the fire village (after you go through the first meteor). I forget the name of it, been a while since I played.
Also, make sure you talk to all the bards in each town/village to get all the songs for the Bard class. And remember to buy all the magic spells (can't remember if you can still buy them all in World 3, buy them as early as possible just in case).
In World 2, the Golem summon is easy to miss. In the mountains above the village with the werewolves (can't remember the name), when you bump into a monster called ????, don't kill it. Instead kill everything EXCEPT that monster as quickly as possible. The ???? monster is actually Golem, and you will get him if you do this.
Can't really think of much else...I've probably forgotten a few things.