Even though I'm new here, whenever I practiced or experimented with a role while writing film scripts, I often realized that sometimes for the accent to work, you might need to phrase sentences differently than how you would with your normal accents. Also, if you're playing as someone from a different nation other than your native one, it helps to know a few different words from that nation's language that stand out. For example, if I were playing as a German, I might say "Ja" instead of just "yes". This especially applies when cursing, for example if you were in a firefight and a few bullets landed nearby, in a high stress situation switching to a native language and cursing is often a reasonable response. Hope this helps!