Katherine didn't speak french exclusively, she used German a lot and Russian on occassion, she even knew some Polish. These leaders were not monolingual,; thus having her speak Russian is fine.
Rat
Indeed. I notice she wrote most of her correspondence in Russian on what concerns internal Russian questions. She even wrote in Russian to her lover and right-hand Grigory Potyomkin.
Also, about the Dutch: Willem van Oranje spoke poor Dutch (he tried to learn it, but he could communicate very well with just High German, which wasn't so different from Dutch and French).
Still ,the Dutch Republic did everything to cultivate the Dutch language and all of the administration was done in that language. Scientific works also often were translated to Dutch from Latin and sometimes written in Dutch first-handedly. Back then however, it was arguebly due to nationalism of course. It was because of geopolitical strategy, not because of emotions. People didn't make troubles writing in every language possible besides that (not in the Republic*).
Also, during the Edo period in Japan. The Dutch were the only ones allowed to trade with the Japanese (they only were granted entry to a trading outpost at Hirado). Until late as the 19
th century, Japanese used Dutch as the diplomatic language with foreign powers (despite the Dutch not being a major power anymore for a long time).
In the Spanish Netherlands Standard Dutch was often persecuted and made a taboo over the Dutch dialects there or French/Spanish, lowering its prestige. Why ? The Catholic Church already in the 17
th century was affraid of Cavinist influences slipping in through that medium. It was no coincidence Dutch-speaking parts and not French-speaking (except for Tournai) revolted in the 16
th century. The influence of language, the power of it was never underestimated.
Change of language is a change of influence sphere, also in the Early Modern Age and also in the Middle Ages, actually. It only became more and more important later on, and it stagnated during the nationalism era. That's true.
*: Not true. When the Edict of Nantes was put through in France, many Huguenots moved to the Netherlands. The Republic did everything to attract them to move to their South African colony. They remembered how hard it was to assimilate Walloons to the Dutch language before. What they did was spreading the Huguenot settlers to different corners of the colony and keeping them as much isolated from one another as possible. Result: after one generation everyone spoke Dutch.
EDIT: Sorry for my long historical rant/wall of text about the Dutch language and history. It's hard to control myself when the subject arises. I always fail at keeping it short.
