Well, I tried the codebreaker code 33000014 0002, and English is on. Speaking of glitches:
Start/Copy/Erase is correct, then German and graphical glitches kicks in. Samus' Daten and two empty fields (English Samus Data and remnants from Japanese's Traditional and Simplified). Both options goes to Start - Kopieren/Loschen) - that's the difficulty (Easy/Medium). There is another German word (or empty field) for Hard, but it's locked by default (can by unlocked by beating the game on Medium or using codes).
Using different values will force different languages (with glitches from the next language (03 - German, 04 - French, 05 - Spanish, 06 - Italian, 07 and above - glitchy stuff).
If at this point the game is playable (with the exceptions of not translated location/difficulty? names on the save screen) from the beginning to the end in English and without glitches, that's good for me. However, fully translated version will be better.
At the end, I have a different question. I'm playing the game on VisualBoyAdvance, and I'm encountering slowdown in certain parts of the game (especially later, with large amount of enemies or large bosses, like Nightmare). Version of the ROM doesn't matter (tried on US and JPN). Zero Mission (other GBA Metroid) works fine. Is there a way to eliminate slowdown?