Where do you see that? AFAIK, Roddenberry’s only connection to Voyager is his legacy of the overall Trekverse. His last direct writing for the franchise was during The Next Generation.
Full cast and crew listing on IMDB. Granted, they’re giving him credit for writing some of that after he died, but he was around for all the prep work for Voyager, and at least the first few seasons. I think you’re confusing Voyager for Enterprise, the series that he really didn’t have any input on, on account of being dead and all.
As a fun bit of trivia, Babylon 5 has a similar issue with Commander Susan Ivanova, the Russian officer who spoke English with a flawless American accent. They hand-wave it by saying that she spent most of her childhood moving from place to place and studying abroad. Her father and brother are both portrayed as speaking with Russian accents, and I can’t recall how her mother spoke in the one episode where she had a speaking part (“Eyes”, via one of the show’s earlier freaky dream sequences)
Of course, she traveled so much and studied abroad as a child because her mom was trying to keep the authorities from figuring out that she was a latent telepath, which would result in her getting drafted into the Psi-Corps
I’m not aware of Jean-Luc having similar issues driving his childhood linguistic development, of course. 
Roddenberry died in October 1991. Voyager first aired January 1995.
Yeah, but Stewart is from Mirfield, West Riding of Yorkshire, and Kemp is from Chesterfield, Derbyshire–a whole 31 miles away! They shouldn’t have sounded anything alike. 
FWIW, my daughter has been perfectly bilingual since the age of 2 1/2. Even though she spent the first ten years of her life in Moscow, going to a Russian school and mostly living with her Russian family, she has always spoken English with a flawless American accent since she learned the language from me.*
*I wish to hell it had been that easy for me to learn Russian! 
Unless, of course, they were both trained Shakespearean actors.
What’s wrong with assuming that he just has a knack for languages, and learned to speak English without an accent?
The Next Generation was the only Star Trek series in first run when Roddenberry died in 1991. It’s possible he was around for the preliminary talk about DS9, but unlikely - especially since he hadn’t really been active in even TNG since 1990, when his health began to fail - and he didn’t live to write for any of the spinoffs beyond TNG. Deep Space Nine started in 1993 (2 years after he’d died), and Voyager in 1995 (4 years after he’d died). (It’s unlikely DS9 or Voy would have gotten made if he’d been alive, honestly, since he had a very specific vision of what Trek was, and DS9, especially, didn’t match it very well.)
Don’t get me started on how everybody from the RSC used to sound the same. There even used to be a show on a local PBS station that featured RSC actors discussing and demonstrating the “correct” pronunciation and phrasing of Shakespeare. Actors I had respected, until then, like Stewart and Jacobi, sounding like jackasses. thinking I think one episode had Brian Blessed. I wish I could remember the name of the show because that could be hilarious.
This is pretty reasonable since he is indeed shown as speaking Latin (“The First Duty”, I want to say) and at least a few words of Klingon from time to time.
Plus, most Europeans tend to learn British English, rather than American, simply because of GB’s proximity to the Continent. To speak it as well as he does, however, would mean that (a) he lived there for quite some time immersed in the language or (b) received some sort of special instruction in it (e.g., studied elocution).
I’ve lived mostly in Russia since 1992, and I still speak the language with a trace of my native accent. It helps a lot if you’re young and your speech habits have yet to become so thoroughly ingrained.
“Jean-Luc” has always been one of my favorite scenery-chewers, and I waited with bated remote control for every episode when TNG was first aired. But at least once an episode I’d wonder “*Why *didn’t they just make him a Brit?”
It was obvious to my young brain that they’d written the part as a frenchman, but then cast Stewart, an archetypal Englishman… and were too lazy to rename him.
I thought I remembered an episode where Data made a passing reference to French as being an archaic or dead language, and Picard became supremely annoyed at this.
Picardy?
That’s it! They confused Picardy with Picadilly.
Why does everyone assume that language spread and accents will remain the same 3 centuries in the future in an Earth which is the capital of a galactic federation?
There’s always the part in Generations when Picard’s fantasy life apparently involved having a family and celebrating an English Christmas. I don’t expect a Frenchman from three centuries hence to be nostalgic about that.
Because in the future France was converted into a penal colony for the New British Empire.