Partial Answer
If we solve the cube with the writing on it we get:
This suggest that each section is a letter in the alphabet, Liam has said also said the alphabet goes in a snaking pattern:
And the pattern:
This is why Y and Z cards are missing, there are only 24 squares
This also means you girlfriend's cube looks like this:
Now the cipher...
We know it can't just be from the cubes because there is a 'y' in the cipher but obviously not in the cubes, (also translating just gives gibberish)