These are the Japanese words which from my experience are most common in conversations between two English speakers who have been in Japan for a fair while:
konbini
manshon/ apaato
chikatetsu
kare raisu
onigiri
keitai
Less common:
jidohanbaiki
Some are quite easy to explain. For example, “apaato” and “manshon” aren’t strictly translatable into Japanese, and rice ball is a horribly clumsy expression for “onigiri”. “Konbini” is probably a combination of being easier to say than “convenience store” and the stores seeming somehow different to and/ or more common than those back home. Why “keitai”, though? It does seem to be the same with use of the word “handy” for English-speaking people in Germany, so maybe it’s something to do with the switchover happening while many of the expats who set the trend already being in the country.
Japan Explained FASAQ 
crella said,
June 8, 2013 at 11:27 am
I don’t know why, but I say ‘keitai’ too (but in Japanese , ‘sumaho’..some kind of linguistic brain quirk). In Kansai ‘jidohanbaki’ is shortened to
“jihanki’ . Do you meet a lot of people who also insert ‘mou’ into conversation?
alexcase said,
June 9, 2013 at 4:55 am
I wonder if it was because at the time many foreigners first came across mobile phones when they were already living in Japan, and it passed from them down the generations of expats here??? (Haven’t even convinced myself with that explanation).
I don’t where I picked it up from, but I quite like saying jihanki in Japanese too. I rarely use either that or jidohanbaiki in English though.
crella said,
June 9, 2013 at 6:27 am
That could be, I bought my first cell phone here in ’98…