One of the stranger anachronisms of Star Trek: The Next Generation is Jean-Luc Picard’s nationality. He is most certainly a Frenchman, a fact established time and again onscreen. This then begs the question of why he’s played by a British actor who speaks with an English accent.
The glib answer is that the character was written before casting had been finalized, and casting the extremely British Sir Patrick Stewart in the role didn’t necessitate a significant alteration from the character’s French roots. In fact, an early episode of The Next Generation helps explain it, turning a minor casting incongruity into world-building canon. It may be a bit of a slight against the French people, but it does answer the question of Picard's accent.
Canonically, Picard was born in La Barre, France, located in the wine-growing region of the country relatively close to the border with Switzerland. His parents were vintners, and The Next Generation, Season 4, Episode 2, “Family” featured the winery where he grew up. The same winery plays a large role in the first season of Picard, as a reminder of where he’s from. Similar details are peppered throughout both shows, such as nursery rhymes he sings in French and his occasional use of the term “merde.”
Certainly, the franchise never shied away from his nationality. Yet Stewart almost never speaks with a French accent, nor is there any comment on his obviously English tone. According to Star Trek: The Next Generation Companion, the original casting call described Picard as “speaking with a mid-Atlantic accent” and whose “Gallic accent appeared when deep emotions are triggered.” The vagueness allowed producer Robert Justman to cast Stewart over Gene Roddenberry’s reservations, as accounted in The Fifty-Year Mission: the Next 25 Years, which further diminished any Gallic overtones to his speech.
The show took care to address the discrepancy, although the way it did so was rather oblique. In The Next Generation, Season 1, Episode 3, “Code of Honor,” Data refers to French as “an obscure language,” which draws a brusque response from Picard. The obvious insult to Gallic pride notwithstanding, the exchange neatly explains away his English accent. If French were an obscure provincial language in the 24th century, Picard likely had to learn English before attending Starfleet Academy, and conceivably learned it so well that he could speak it without a trace of French.
Indeed, the notion holds up, even as the concept is stretched further. As a little-spoken language, French might not work with the universal translators, for instance, which explains why Picard mostly speaks English but occasionally lapses into (untranslatable) French. It also adds weight to his ambivalent attitude towards his family, who wanted him to work the vineyard and all but disowned him when he left for Starfleet.
Nevertheless, it’s become an odd but endearing quirk to the character, now so utterly bound to Stewart that it’s impossible to imagine anyone else of any nationality playing the role. To its credit, the franchise never forgot his roots, which makes the lack of native accent a good deal easier to overlook. Somehow, he wouldn’t be Jean-Luc Picard if it were any other way.