I think you are mistaking logics of pre-nation state empires and later modern nation states.

Compare the Ottoman Empire to modern Turkey, or the Austro-Hungarian Empire to modern Austria (and Hungary), or the Russian Empire (West of the Urals—East of the Urals there’s a bit of different, colonial logic) and contemporary Russia. In all those cases, you see massive size differences. Why? Because empires, including the Grand Duchy of Lithuania, were by their nature multi-ethnic, and the rulers would not necessarily speak the same language as the ruled. In many Eastern European “national awakenings”, the parts of the elite abandoned their previous language and adopted (a version of) the language of “the people”. This was notable especially in Hungary, Finland, and Turkey, where many of the elites spoke Dog Latin, Swedish, and (heavily Arabic and Persian influenced) Ottoman, respectively.

This seems very different from the modern logic of nation-states, where the governance unit (general the state, or in some cases the sub-national unit like German-speaking South Tyrol in Italy or Catalan-speaking Catalonia in Spain or what have you) and the cultural area are congruent. And if they’re not congruent, there are often movements to politically or culturally include the exclave (think the Hungarian diaspora in Romania, Serbia, and Slovakia, or the Russian diaspora in their “Near Abroad”, like eastern Ukraine or the Crimea) and/or politically or culturally nationalize the enclave (see the problems Spain is having, the demands of the Baltic states to on their Russian-speaking semi-citizens, the periodic banning of the Turkish language in Bulgaria under Communism, etc.). There are also those who except just a separate status for certain areas (South Tyrol in Italy is a good example, the Swedish-speaking Åland Islands in Finland another).

Many states that we think of as cohesive are the result of direct national policies in the modern era. Eugene Weber has a great book about France called Peasants into Frenchmen where he argues as late as the 1880’s, 40% of France didn’t speak French (they spoke instead Breton in Brittany, Basque in the Pays-Basque, German in Alsace, and a Langue d’Oc dialects more often similar to Catalan than Parisian French like Provençal across the South). It was things like mass culture enabled by mass communication tools like the printing press and the telegraph, mass conscription into the army, a national economy enabled by roads and railroads, and above all a national public school system that brought all those within the French state into the French nation. If this was just possible in France (one of the most technologically and economically developed areas of Europe) in the late 19th century, it wasn’t possible in Eastern Europe even if there had been a will to do so (I don’t think there was as that just wasn’t the logic of even modern inland empires for the most part, with a few obvious exceptions like the British with their “Celtic fringe”).

Lithuanian, for instance, wasn’t even a written language until the 16th century, and many of the nobles (as in other empires) were not native speakers of the dominant language, but rather spoke local languages like Ruthenian. Even if the Lithuanian Grand Dutch had lasted until the nation-state logic began replacing the empire logic in the 19th and 20th centuries, many of these once politically Lithuanian notables would like have allied with their local co-linguists (the logic of nationalism is often linguistic based, with other facts like territory, historical government, and religion usually coming second). In the Grand Duchy, laws were multi lingual, for instance, and there was really no pressure on the mass of conquered Ruthenian-, Polish-, Latvian-, whatever-speaking peasants to adopt the Lithuanian language. In fact, the opposite seems to have happened, and many once Lithuanian speaking lands gradually got Polonized. By the end of the 17th century, as far as I understand, the language of court was predominantly Polish rather than Ruthenian, though I don’t know enough about the internal politics to know exactly how this process happened. I’m not sure when Lithuanian was replaced by Ruthenian as the dominant language of the court and administration, but it seems to have been before that, maybe around 1500. Lithuanian remained throughout its entire existence commonly spoken in a large around Vilnius and nearby governorates, but not across the Grand Duchy generally after it was replaced by Ruthenian. Such language change isn’t unprecedented: remember that the English elites were once Norman speaking, the Finnish elites once Swedish-speaking, and the Hungarian elites once Latin-speaking, all languages adopted slowly as elite languages. This elite adoption of prestige languages is quite common globally, throughout history. For instance, one of the reasons that Madame Nhu, the de facto First Lady of South Vietnam was so hated was because she spoke and wrote Vietnamese rather poorly (apparently), having had all her formal education and much of her social life in French. Going back, David Anthony suggests in The Horse, the Wheel, and Language this elite adoption is how Indo-European spread from a language family of the Ukrainian Steppes to the dominant language family from Spain to India. However Lithuanian the Grand Duchy was in name, it wasn’t necessarily as Lithuanian in language throughout its existence.

So, in short, not only do multi-ethnic empires not usually spread their language to their peripheries like language nation states do, but even if the Grand Dukes had wanted to, they didn’t have the state technologies like mass public schooling to do so, and even if they wanted to and did have the technology, they were mostly conducting their business in Polish or Ruthenian, rather than Lithuanian, anyway.