From signing the Treaty of Brest-Litovsk until the end of WW1, several new states in Eastern Europe were created. I’m interested in why were the countries in the Baltic split between Lithuania and the United Baltic Duchy, instead of leaving Estonia alone since it speaks a completely different language.
Were the areas mostly comprised of Russian-speaking citizens and thus the language didn’t matter? Were there geographic reasons leading to tis decision? Or something entirely different?
The reasons go back to the 12th and 13th century. Namely the Baltic crusades, in which christian entities of Europe vied for power in the Baltic region, that was one of the last areas in Europe, that were still "pagan" i.e not Christian. The destinies of Lithuania and Latvia and Estonia diverged considerably during that period.
Namely, while the provinces of today's Estonia and Latvia - back then provinces of Estonia, Livonia, Curland, Latgalia - became under the rule of different western countries. Northern Estonia was conquered by the Danes, and the rest of Estonia and Latvia became under the rule of Livonian Order - a theocratic military order comprised of Germans (both nobility and farmers - all looking for better opportunities in the east). Even though the rulers of the area changed throughout history, the local rulers - nobility - were always Germans (and some Swedes and Danes etc). They are known as Batlic Germans and their local autonomy lasted untill 1917 revolution in Russia - for nearly 700 years the lands were effectively ruled by German-speaking nobility.
In Lithuania, however, the locals managed to win against the crusaders (at a very high cost of course) and eventually converted to Christianity themselves and expanded their power to the east. Lithuania was at a certain point, one of the largest countries in Europe by landmass. Later, Lithuania joined with Poland in the Polish-Lithuanian commonwealth, untill the dissolution of the state in dumerous participations. But the point in this case is: Lithuania never had a sizeable German minority and that minority wasn't in a position of power.
So when the Russian Empire collapsed, the United Baltic Duchy was (inteded to be) created in the provinces, in which the German nobility had always held power. There simply was no such contingent of people in Lithuania.
And one of the reasons why this new united duchy would not succeed, is indeed the fact that the land they would've ruled, was majority Estonian and latvian, and Germans only numbered a small minority (a few percent). With the collapse of Germany and the rise of nationalism in the Baltics, it was just not viable, to have a German led state in the Baltics.
(To anyone else who was confused, the title means that Latvia and Lithuania speak languages similar to each other, not that their languages are similar to Estonian. Estonian’s only close linguistic relative is Finnish.)