I expect if the "American" character abroad has a superior attitude toward foreigners, an "ugly American," American idioms might fit the bill. One of my sisters-in-law speaks English as a second language. She had a difficult time when we first met understanding me because I frequently speak in idioms. I toned them down once I realized she couldn't process them. She spoke English fluently, but my brother still had to translate my speech for her. She's lived in the U.S. now for twenty years and understands me easier. Her native language doesn't have many conventional idioms, not the way English does.
Register is in play. Formal to informal to outright casual, superior to subordinate, subordinate to superior, peer to peer. In some subcultures formal address is considered a sign of respect. In others formal address is considered insulting. There's a fine line between informal address acceptability and casual address insuitability. Some subcultures demand acknowledgement of station between participants in a conversation. In other subcultures, some people are honored and flattered by being accorded equal status when they believe they're below the station of who's addressing them, to illustrate a few areas of register as a facet of voice. There are formal situation idioms, informal idioms, and casual idioms too.
If a character's personality and behavior is firmly established, getting into the register facet of voice of the character for draft writing might help bring out voice. If not as well established as might be desired, then marking stand-in words or phrases for reconsideration later is a best practice to keep writing flow moving. During rewriting when a character's nature has become fully realized, they're easy to go back to and contemplate for diction revision. They're likely to be more natural and less forced seeming that way, too.
In foreseeable and unpredictable ways, a formally speaking American abroad could get into all kinds of entertaining trouble from causing misunderstandings. Keeping track of the alien nuances of station and position and decorum might cause a decision to default to purely formal address in all situations. It's safest but far from perfect, but could be a perfect decision for fiction purposes.
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