There’s lots of English slang in spoken Icelandic, and a bit less in written. (EDIT: Please see Alex’s comment for notes about “cognates” and “false friends”, etc. as well as good examples!)
In person Icelanders, when chatting with friends, might use straight English words for things that may not exist in Icelandic, that have a more complicated Icelandic word, or as some sort of in-joke. I’ve also heard a lot of phrases both said and (casually – like on Facebook comments and forums) written entirely in English in the middle of an Icelandic conversation. In books people tend to use more proper Icelandic.
Example: Páskaöl: basically Jólaöl nema bara í öðrum umbúðum. / Easter (that orange soda and malt drink I mentioned before): basically (Christmas version of the exact same thing) but just in different packaging.
“Lengsta “alvöru” orðið (as in eitthvað sem er actually…” / The longest “serious (real)” word (as in something that is actually…”
Here are some more examples, sorry but I don’t know the most common/useful words so I just listed random ones (note that if your dictionary says something else I’d trust it instead – the online dictionary I use can sometimes be off):
fésbok – Facebook
júróvision (also “Söngvakeppni”) – Eurovision
sjoppa – shop (mini kiosk, etc.)
sorrý – sorry
bingó – Bingo
krem – cream mixed with something like eggs, colouring, flavouring (ex. chocolate mixed in with cream is súkkulaðikrem), also it means “paste” in toothpaste, it can mean ointment, etc. As far as I know we don’t have a single word for this.
massi – mass
stíft – stiff, stubborn, stand-offish
smyrja – sounds similar to”smear”, spread, grease, butter (but apparently can mean doing it thickly too, ex. layer frosting over a cake)
konfekt – confectionary
pottur – pot, liter, chamber pot, hot tub
binda – tie, bind
karamella – toffee, caramel, also used for chewy flavoured candies with the consistency of either of those
tóbak – tobacco
gólf – floor
vanilja – vanilla
djók (also “grín”) – joke
prenta – print
bókband – book binding
Bann – ban
Smella (only when talking about online, it has other meanings) – click
Screenshot from hugi.is, as you can see some people like to use casual, random English – but this doesn’t mean their thread is in English. To illustrate what I mentioned above, “folder” (even the “computer folder” meaning) has an Icelandic word, so does “awesome”, and certainly “anyone” does too. Sorry but I don’t have more examples for you because most things I have saved were written by friends and I don’t want to copy them even if it’s not private.
Here is a link to Snara.is’ online slang dictionary. There was also at least one slang dictionary published but I hear it’s bad and out of date.