Yoroshiku onegaishimasu, just because there is really no equivalent in English and it is used in so many situations, and yet it's not hard to know when to use it. I love words that don't have a direct English translation.
Because I expose myself to so much Japanese media I find myself using Japanese words and phrases in my head so often even though my actual knowledge of the language is poor. Haven't said any out loud, thank God, but my dad says that sometimes I say things with weird intonations. I think it's because some days, especially when having a drama marathon, I actually hear more Japanese being said than English throughout the day.
...I do find myself saying "Eeehh?!" sometimes, though, but no one has ever said anything about it. While actual words never slip out of my mouth, certain sounds that Japanese speakers make do. I also find myself making that "n/m" sound instead of saying ok/yeah/yes, something that no one (around here) really does. I've never said "hai" out loud but it is one that I often say in my head. When I'm alone I will sometimes mutter a "yosh(i)!" to myself in order to motivate myself.
It's a problem, haha.
