Dialog and Accents in Spymaster's Lady

Early on in writing, I knew I was going to have my folks speaking French for half the story and English for the other half.
How to handle this?
I didn't want anybody's internal 'voice' to change in the middle. That was just a non-starter. No can do. The character HAS to sound like himself.

So I gave everybody their 'native voice' from the start. Whether they're speaking English or French, their voice stays pretty much the same.

Picking what the voice would be took a little head-scratching.

Grey speaks and thinks in standard English. This one was easy.

Adrian is also standard English, but he has the slightly formal register of someone who speaks completely fluent, accentless English, but as a second language.

Now Doyle gave me hours and hours of agonizing.

Doyle mostly chooses to speak Cockney-with-a-bit-of-rural dialect in English. But what to do with him in French? I couldn't have him speak 'standard English' when he was in France and then suddenly go Cockney the minute he set foot in England, could I?
(thinkthinkthinkthinkthink)

So I wrote him as Cockney all through. And I described him as having a Paris-argot-with-a-bit-of-Breton dialect, which would be the equivalent.

This is a weird choice. Sometimes you just have to pick something and go with it.

Annique's voice is French in cadence and word choice, of course.

My major problem was always deciding 'how much' to do this. Enough to make the point. Not enough to intrude. On revision, every line was a conscious decision that called for a bit of twitchy adjustment.

Her spoken language when she first lands in England is the most 'French' she gets. I'm trying to show her 'translating' in her head for a chapter or so.

As to creating Annique's voice... Mostly, it just comes natural. That's the best way I can put it.
This is what Annique 'sounds' like in my head.
This is how I hear her. I do speak workaday French.

Especially in revision, I'd 'parallel write' -- thinking of both the English and French of the sentence and shifting the words to give it a French cadence.

And I spent a long time listening to French folks speak English and dissecting how they sounded.

But by the time I came to write Annique, I had the voice inside me. It's not translation. It's the old 'voices in my head' that is either madness or being a writer. (Or, of course, both.)