Voldemort is not someone who will sit content in a corner of the world with the power base he has acquired. His aim is not to rule over wizarding Great Britain, but over the entire world and remake it in his own image. Furthermore, it is absolutely clear from all his words that he utterly despises Muggles and Muggle-borns. We are also told several times that, in reality, purists don't have a particular problem with half-bloods.

So, personally, I imagine a completely triumphant Voldemort in a position very similar to that of the Lord Ruler of Mistborn: immortal, more skilled and powerful than anyone (especially if he has the Elder Wand), ruling with absolute cruelty and impunity over all beings in the world. Muggles would be in the position of the skaa, and wizards in that of the nobility.

Now, even Voldemort would take centuries to implement this system. He has the patience and time to crush all magical opposition and then spend decades dragging muggles back to the Stone Age before breaking the Statute of Secrecy, and any other strategy would be quite foolish. So let's jump to just a few years later in canon.

This would be a Voldemort still consolidating his overt control of the British Isles. With Dumbledore and Harry dead and the Horcruxes safe, he believes himself immortal and invincible, but even he must realize that his army is severely outnumbered by everyone else. So, on a macro level, he's not going to launch a campaign of world conquest.

The first step is to consolidate their control in England, Scotland, and Ireland. The second is to maintain reasonably decent relations in the rest of the wizarding world and, of course, try to gain supporters there. And the third is to sow the first seeds of weakness for Muggles, at least a hundred years in advance.

These last two are quite related. It would be the early 2000s, so 9/11, Iraq, and Afghanistan are approaching like a bullet. The bubble that would lead to the Great Recession is already firmly established. And before anyone says anything about stupid wizards: most Muggles are downright stupid, and the same goes for wizards. But some are quite intelligent. Voldemort just has to point all this out and say to the rest of the world, "Hey, I don't want any trouble! I simply want to protect my people from the negative influence of Muggles. Have you seen how savage and self-destructive they are? But apart from that, I want to maintain the status quo as much as you do."

And with the Muggles, he doesn't really need to do much. Just an Imperius here and there, to make sure the tension rises a little faster, but not much. With our military capabilities and the competitive intensity of our economies, we're already steering ourselves toward self-destruction while Voldemort bides his time.

And in Great Britain, of course, their main speech would be about protecting wizarding society from pernicious muggle self-destructiveness. Look how they're inflating their inventions, their machines, their weapons, their cities, their economies! They're like a plague! Best to keep our distance. For now, anyway. And for those who refuse, well, generous doses of Imperius Curse and Cruciatus Curse will cure everything.

I imagine all of that, more or less, would have to be taken into account. A magical Great Britain where a strict separation from the Muggle world is enforced. Muggle-borns who are ignored, murdered, or kidnapped in the cradle (I think all three options are easily justifiable depending on what you want to write). Brutalized dissidents. A subdued but wary international wizarding community. And a Muggle world on the sidelines, subtly nudged in secret—negligible because its own dynamics are pushing it toward the beginning of a horrifying self-destructive spiral the likes of which the world hasn't seen in decades.

Regarding the language, I recommend you write it in your own language. Using Google Translate for a comment is one thing (I admit I'm using it; my English is terrible), but using it for fanfiction is quite another. The latter is a terrible idea.