My players are curious. Here in Spain only the 1st ed got translated, and that is the one they have. It seems to me they are 99% compatible but can someone elaborate on the differences?

Well, first thing's first - check out 'D6 Space Opera'.  It's the 'most recent' version of the D6 system, and is OGL now, meaning you should be able to get it everywhere for free.  In fact, I think that drivethrustuff has it still.  If not, Google surely does, and I think even in tranlations.

Assuming you mean the differences between 1st edition and 2nd Ed Revised and Expanded, let me see if I can do this off the top of my head...

1st to 2nd:

1) Scales were introduced.  Basically it meant that character weapons weren't effective against vehicles, star ships couldn't hurt the Death Star, etc.  As I recall, 1e tried to handle this via large dice pools on the few ships they did stat out.

2) Skills were revised a bit.  Also I believe the system for advancing attributes was introduced.

3) Force Powers were reworked.  Particularly along the lines of cost.  In 1e (IRRC) they were skills, in 2e Attributes at a discount.

That's all that jumps out at me.  I think maybe they re-did the droids, too, but I'm not sure.

2nd to RE:

...honestly I can't think of anything.  The 'expanded' part was certainly true.  You no longer needed odd sourcebooks to cover most of everything you saw in the original trilogy, because RE had all those stats.  But if you had those books already, I don't remember gaining much.

Changes in later editions include adding the Wild Die and lot's of tinkering with skills, scale and various defence rules and lots of stuff about turn sequence (reaction rolls and hastened actions). The latter from my experience is something everyone got wrong anyway. It is only rereading the rules recently in these more mechanics conscious times that I finally figured out how the turn sequence  with multiple segments was really meant to work. And no I can't really explain it because I've already forgotten. :-)

Didn't matter really. You play WEG Star Wars to roll tons of d6 and go nuts; don't sweat the small stuff.

PS:
That said, if cleaned up rules are your thing I will echo the suggestion of checking D6 Space (and Mini Six ) for inspiration.

Some earlier discussions:
http://www.therpgsite.com/showthread.php?t=26196&highlight=star+wars
http://www.therpgsite.com/showthread.php?t=23571&highlight=star+wars+revised

There are some mechanical distinctions between 1st and 2nd: the Wild Die, skills became less cohesive, vehicle speeds became concrete rather than relative, etc.