4 ideas
Johnny Porter is a Gitksan Indian from British Columbia and an Oxford-trained linguist fluent in many languages, including Siberian native ones. He is recruited after a scientist trapped in a secret Soviet research station in the Kolyma region smuggles out a message asking for him by name.
Colonial and imperial administrations tend to see minority natives as interchangeable, low-status people who are not worth scrutinizing. An operative who can pass as a member of such a group gets freedom of movement that no forged document could provide, because officials look straight past him. The prejudice of the powerful becomes the gap in their own security.
Papers can be checked, but a native command of dialect, idiom, and minority languages signals belonging below the level of conscious verification. Listeners grant trust to someone who sounds like an insider before they think to question him. Deep language skill is therefore a more durable cover than any forged identity, and gaps in it are the likeliest way to be exposed.
A long infiltration is built as a chain of separate identities: a foreign sailor for the port, a local laborer for the region, a trusted driver for the facility. Robustness comes from segmenting the journey so that no single cover has to bear the full weight of the mission.