We're already starting a nice list in Spanish, what about French, German, Italian, Portuguese?
Well, in German what is used most are the English internet slang words and abbreviations.
There are only a very few that I know of which are originally German, as follows:
- DAU Dümmster anzunehmender User = the-silliest-possible-user-one-could-(and-should)-imagine (formed after "GAU" = "grösster anzunehemder Unfall" = "worst-case accident scenario" which could be for example a melting core of an atomic power plant)
- LG = Liebe Grüsse = Greetings/ Best Wishes/ Cheers/ ... (whatever your preferences are; it's just a simple greeting)
- mfg = Mit freundlichen Grüssen = the same as LG but more formal
- KA = keine Ahnung = no idea at all
- SIG = Signatur = signature
- WE = Wochenende = weekend [<- this one actually could be an English one - WeekEnd ...]
- vllt = vielleicht = probably
- g (or) gg (or) ggg = grins (= what is written in cartoons to signify that a person is grinning; and: the more 'g', the more laughing); also many versions for mischievous grins (fg), etc.
- HDL = hab dich lieb = love you
- LMAA = leck mich am A**** (vulgar) = FU in English
The ones which are really rather universal and widely understood are 'g', 'mfg', 'LG', 'WE' and probably 'KA' - these also might be used for SMS communication.
(Of the
German Wiki list I took only the ones I came across already as I have no way of being sure if the other are authentitc, or restricted to certain forms of chat only. You'll be surprised to know most from this list, as they are English.

Certainly there exist a few more (German ones!) than listed here; these are only the ones I remember right now and have found on the www for confirmation.)