I have used all common terms, slang terms and derogatory terms in the various campaigns that were set in Asia, the US, Europe and the Middle East. As a GM or a player it all depends on the age and social level of the character/NPC you are playing.
My current campaign, I have a German fieldgrade officer who is well educated and a decendant of Prussian Aristocracy. He sees the current conflict as a continuation of past conflicts and the presence of the "evil "Bolshies" on German soil as a personal afront. He does refer to them as Bolshies normaly.
Ivan, is a common one, Reds also, these are used by PCs who are leftovers from the Cold War. A more modern conscript who grew up after the evil empire fell may revert to "Ruskies" or "Russians"
Another term I have used is "W.P.'s" a throwback to the Cold War for "Warsaw PAct"
Speitzi's for Speitznatz as I used in my idea for a comando attack on Oz in that thread.
Oh yeah, I also refer to them as "The Bear" as well usualy when talking about a larger organization or nationaly. "Well, then the Bear went and rolled into Berlin."
Kraut and Hunn for Germans, Red Hunn is a term I used for East Germans.
Pole or Pollock for General Polish forces,
I also use the term,
"T-shirts" or "Red Berrets" or even "Blue" for their Paratroopers and Spetizis, and it is usualy followed by "T-shirt" or "striped T-shirt wearing MFers" or C-suckers" or similiar epitath.
Koreans and Chinese <in my T2K World the Chinese are our enemies as it is in the real world. And China is in another Civil War with a faction supported by Taiwan and the US. So the normal "Gook, Slope and Red" For the Chinese they also get "Chink and Chicoms" I also toss out "Moas Boys" or for Korea "Kims People" I have also used "Kims" to refer to Koreans. Although I have also used the term "Maoists" once for Chinese too, as well as PLA or "plas" <peoples liberation army> with an "s" for plural.
Mexican forces are the standard but I also use "Mexis" quite frequently, with the standard racial racial slurs to riendforcement. Sometimes they will be called "Federalis" to denote between bandits, mauraders and the Mexican Army or Government Forces."
And of course I also use the term Polar Bear or Southern Bear, Polar Arctic Forces and Southern or even "Lost Bear" to refer to the forces from Cuba or "Little Bear from Cuba" a line stole from the the TV Series "I Luv Lucy" when they bought a dinner. But it also denotes the small size of the force who will in all likelyhood not survive.
Middle East,
Hadjis, Achmed, Achbar, Camel Driver, Camel Jockey, Sand Niger, Carpet/rider/salesman, Sand People are all some terms of endearment for the wonderful peaceluving peoples of the middle east who are you enemy.