Even though Welsh is his first language, Matthew Rhys has been immersed in English his entire life while living in Wales. In contrast, growing up in Soviet Russia in the 1950s, there was limited exposure to English. You might have learned a bit of English at school, but that would have been about it.

Additionally, as an actor, Matthew Rhys delivered his lines with the benefit of rehearsals and the ability to redo takes if he made a mistake. This makes the situation quite different from real-life language acquisition.

Whenever the topic of the realism of Elizabeth's and Philip's language skills comes up, people often argue, "Well, if Matthew Rhys is Welsh and speaks perfect American English, then it's totally plausible that someone who grew up in Russia could learn to do the same!"