Journals from the 18th century have no substantial differences from what?
The consensus among linguists is that modern, standard Czech originated during the eighteenth century. By then the language had developed a literary tradition, and since then it has changed little; journals from that period have no substantial differences from modern standard Czech, and contemporary Czechs can understand them with little difficulty. Changes include the morphological shift of í to ej and é to í (although é survives for some uses) and the merging of í and the former ejí. Sometime before the eighteenth century, the Czech language abandoned a distinction between phonemic /l/ and /ʎ/ which survives in Slovak.