The Thirty Years War is in fact a series of several wars fought in parallel or in succession. But it started out as an inner German civil war, and until 1631 it mainly was. Bohemia was multilingual, true, but it was a domain of the Habsburgs, which in turn were Germans. The mainly protestant Bohemians seceded from Habsburg and choose Frederick V, Elector Palatine in 1618 as their new king, who was German too, which started the war (and caused Frederick V to be called "winter king", because his reign endet in the Spring of 1619). Then Germany was split into two alliances, the League (the Habsburgian Emperor and mainly catholic German countries) and the Union (the protestant German countries with the exception of Saxony).
The League tried to conquer the Union states with two armies, the Bavarian army led by Gen. Tilly, and the Imperial army lead by Gen. Wallenstein (Waldstejn), who was Bohemian despite his German name. They devastated most German protestant countries till 1631, when the King of Sweden entered the battle to "save Protestantism".
When France entered the battle, it was trying to oust the Habsburgs from Spain or at least to weaken the Habsburg influence in Europe.