If the aim is to make leaders feel realistic, Firaxis has done a terrible job, or as you fear "a ruin to the series", both because of the awful leader backgrounds (which do the opposite of adding atmosphere), and many of the leader voice actors having terrible delivery (Robert the Bruce hasn't much energy).
Not all of us English speakers will be taken in by bad voice acting just because it happens to be in the correct language. As earlier pointed out (by
Siptah and others), plenty of native speakers of other languages have mentioned issues with leaders from their nation speaking their language terribly. Danes pointed out how cartoony Bluetooth in V sounded, Dutch people objected to both voice acting in V and VI, and so on. Frankly it seems many voice actors are hired just because they can speak a language, not because they can speak it well with good inflection. And we all remember the awfully meme-worthy delivery of Elizabeth in Civ V (and the frankly similarly unimpressive delivery of Victoria in VI).
Having accents for other languages or speaking other languages is all well and good, which is why accented English (if done properly) won't be a problem.
Re: Napoleon speaking English, that would only be encountered if you were not playing as Napoleon. The troops would speak French when under your command.
A fellow appreciator of the Age III voice acting, welcome!
And yes, it's true VAs get away with that quite often. Daniel Day-Lewis played both Lincoln and a hard-boiled American oil man because of his acting abilities (he's British). Similarly many professional VAs can cast a nice illusion using pitch and tone in ways that a regular person (like the academics Civ V favored, for example) simply don't know how to use properly. When I heard Tokugawa and Kangxi in Age III, I was under the belief they were Asian-American actors until I checked the credits.
A mixed English/native language approach would work well I think, and as suggested earlier I think it would strike a nice balance between good voice acting and native speakers.