The first big difference I notice entering a foreign country is: the language. Eventually I compare the way I was brought up and what's familiar to me, with the new things I encounter. In my opinion, you keep comparing for a long time to be aware of the effects. I still catch myself liken Americans and Germans.
Inevitable the language has a huge effect. I had English since 5th grade, which was very helpful of course, but nevertheless it is totally different than the actual spoken American-English. Back in the 80's Germany was oriented on Great Britain so I learned countless British expressions and idioms like "trousers", "lay the table", or "bloke". These words weren't hindrances but rather involved me in conversations with people I coincided. When I write an essay I exercise the unpractical approach of first thinking in German and then trying to find a way to translate or to describe it in English.
Living in a foreign country also has an impact on my native language: it gets less used. As a consequence it is harder to recall the daily used vocabulary as quickly when talking to my relatives and friends back home. Even the grammar and the spelling get out of hand! With a sister who is very fond of German, more than familiar with its proper usage and the correct orthography, the gradual decay of my mother tongue is thankfully, and sometimes unasked, impeded.
Introducing German words into the English vocabulary is very useful at times. It is not essential but makes things less complicated. "Ohrwurm" for instance, which translates as "earworm", refers to those songs that you hear once and they stay lodged in your brain all day.
Vice versa there are also expressions and idioms in the English language usage...