I know Matthew Rhys's (who portrays Phillip Jennings) first language is Welsh, and speaks English with a Welsh accent, so he's imitating the US accent in The Americans. But US-born actors with American English as their mother tongue, portray many KGB spies like
his movie and real-time wife, Keri Russell, who portrays Elizabeth Jennings.
Frank Langella who portrays their first KGB handler. Margo Martindale who portrays Claudia, the Jennings' second and fifth KGB handler.
The Jennings never lived in the US when they were kids, for in the scene (please edit this if you remember which) where they're checking into their motel, the air conditioner dazzles Elizabeth who remarks the USSR doesn't have these luxuries.
The Jennings are flashed back as impecunious when they were kids. So they probably didn't learn English diligently until they joined the KGB in their 20s. So it feels unrealistic that these KGB spies could've acquired US English to speak like native speakers. Research on L2 (second language) acquisition doesn't substantiate such success.
Fromkin. Linguistics: An Introduction to Linguistic Theory (2001 1 edn). p 546.
A strong foreign accent can often persist even after years of practice.
Yule. The Study of Language (2016 6 ed). p 210. If anyone has the 2020 7 edn, please don't hesitate to edit this.
Despite the fact that insufficient time, focus and incentive undermine many L2 learning attempts, there are some individuals who seem to be able to overcome the difficulties and develop an ability to use the L2 quite effectively, though not usually sounding like a native speaker.
However, even in ideal acquisition situations, very few adults seem to reach nativelike proficiency in using an L2. There are individuals who can achieve great expertise in the written language, but not the spoken language. One example is Joseph Conrad (1857–1924), who wrote novels in English that became classics of English literature, but whose English speech retained the strong Polish accent of his L1. This might suggest that some features of an L2, such as vocabulary and grammar, are easier to learn than others, such as pronunciation. Indeed, without early experience using the sounds and intonation of the L2, even highly proficient adult learners are likely to be perceived as having an “accent” of some kind.
The optimum age for learning may be during the years from about ten to sixteen when the flexibility of our inherent capacity for language has not been completely lost, and the maturation of cognitive skills allows a more effective analysis of the regular features of the L2 being learned.
Some KGB officers speak accented English like those portrayed by USSR-born actors, like
Lev Gorn who portrays Arkady Ivanovich Zotov, the KGB's Rezident at the Soviet embassy.
Costa Ronin as Oleg Igorevich Burov, originally the Soviet embassy's Science and Technology officer, a privileged son of a government minister.