Looking up the definition for expat, you get:
noun: expatriate; plural noun: expatriates
a person who lives outside their native country.
"American expatriates in London"
synonyms: emigrant, non-native, émigré, migrant, economic migrant, guest worker; More
displaced person, refugee, exile;
"the level of salary paid to expatriates working overseas"
denoting or relating to a person living outside their native country.
synonyms: emigrant, living abroad, working abroad, non-native, émigré; More
displaced, refugee, exiled;
antonyms: native, indigenous
verb: expatriate; 3rd person present: expatriates; past tense: expatriated; past participle: expatriated; gerund or present participle: expatriating
send (a person or money) abroad.
"we expatriated the prisoners of war immediately after the end of the war"
synonyms: settle abroad, live abroad, relocate abroad More
So my suggestion:
We deal with problems uniquely facing those persons living, studying or working outside their native country
I specifically chose "uniquely" there to show that it needs to be for expats only - eg local tax issues affect everyone, not just expats, but inter-country non-residence tax issues affect expats.