Arthur speaks foreign languages exactly as he's been taught (courtesy of Uncle Sam). He remembers his conjugations and has studied all of the relevant vocabulary. At best, he sounds like American-accented text to speech. At worst, he's picked up a few odd artifacts from his textbooks - in some languages he's excessively formal, in others he sounds somewhat effeminate, in others his vocabulary is 50 years out of date. It's enough to manage a job with just a few odd looks.
Eames has learned all his foreign languages from pubs and hostels and the occasional bedfellow. His accent is fantastic but extremely local to wherever he picked up the language, which makes people laugh when he goes elsewhere. He regularly forgets vocab and has to resort to mime (thank god he can act). He does however know every single swearword. He gets by well enough to travel under the radar.
At some point over the years they’ve spent doing jobs together, Arthur started filling in the blanks for Eames - cutting off his "um"s and "you know, that thing"s with the correct words, finishing his sentences properly. They work together well like that - Eames builds fluent rapport with their target while Arthur focuses on getting the specifics of what they need.
Their team members finally realize something is up when they start doing it in English.
