It's not really intentional, but I often say certain words in different languages, just because I forget the English word for it. For example, thinking I just said "thanks," I realise afterwards that I say "danke." I do this for pretty much any language I know any words in, which at the moment is mainly German, French, Spanish, Japanese and, more recently, Korean. What confuses me is that I can barely speak basic French and almost none of any other language.
As for slang, I often use what some would consider American slang, just because so much of British TV seems to be American, and some slang that me and my brother made up or got from the MBMBaM podcasts.