In OTL almost all alphabets of Slavic nations using latin script were based on Czech alphabet. What POD is needed to change it, so at least some of these Slavic nations use alphabet based on Polish, so w instead v, no háčeks and more diphtongs (perhaps Croats and Slovenes, bonus if Slovaks are included and respect if Czechs too)? My thoughts are:
-erode Czech alphabet from use if possible (forbidden as"heretic" and replaced by new one based more on German, during national revival Czechs adopted more "Slavic" script from Poles)
-preventing partitions also would be helpful. As result Polish culture fare better and is more influental. If 19th century Poland is succesful state Croats and Slovenes may see Polish as example how alphabet of Catholic Slavs should look like.
-erode Czech alphabet from use if possible (forbidden as"heretic" and replaced by new one based more on German, during national revival Czechs adopted more "Slavic" script from Poles)
-preventing partitions also would be helpful. As result Polish culture fare better and is more influental. If 19th century Poland is succesful state Croats and Slovenes may see Polish as example how alphabet of Catholic Slavs should look like.