Your English will change
Even if you haven’t been studying Japanese – we’re all guilty of it – just by living here, your English will start leaning towards, well, Japanese-English. Salaryman will replace businessman, slap some egg salad on bread and call it a sando, and you’ll naturally tack on an ‘okay desu’ at the end of your sentences.
It’s surprising to see the number of loan words from other languages used in Japanese, although not all of them have the same meaning here. Of course, once you get some language skills down, you’ll start inserting those untranslatable Japanese words into your everyday vocabulary. Just make sure to watch your language when you leave Japan.