In WWII, a joint Australian / British force carried out an attack, posing as Japanese fishing boat, and sailed right into Singapore harbour to place explosives

In WWII, a joint Australian / British force carried out an attack, posing as Japanese fishing boat, and sailed right into Singapore harbour to place explosives on the vessels there. They flew a Japanese ensign, wore sarongs and wore tan makeup. Operation Jaywick was not a huge strategic success (and the local population was subject to reprisals since the Japanese thought it was their fault) but it did raise morale a lot in allied forces, as it was an early blow against Japan (which had seemed invincible at the time). Even in the extreme example of white Australians trying to pass as Malaysians, special forces have pulled of plenty of raids without the need for native language speakers. Even if you need someone highly fluent who can pass as a native, most of the time there's a nearby country where they have some kind of grudge against the belligerent. I can think of a lot of potential theatres where finding an enemy of a belligerent who can pass as a "native" would not be difficult. North / South Korea, China / Taiwan, The Middle East ... conflicts often occur in places where there's a lot of conflict. Also, in a war, often the military and civilian sector are stretched thin. Russia can't spare the troops to guard everything as well as they could in peacetime, and even if they could search every vehicle they can't afford to gum up their logistics.