this is the alliance
alliance = {
id = { type = 9423 id = 4 }
type = militaryalliance
expirydate = { year = 1431 }
participant = { LIT POL MIN}
}
and there is no silesian culture, there are not enough cultures, if POL get silesia it converts to polish culture
So I suggest to swap LIT POL to POL LIT
Siliesian culture can be found in MyMap if I recall corectly, so it is possible.
It does not convert, I tried it with capturing province "Oppeln" for Poland and the culture remained "german". Or maybe I need to play longer, the event may come later (?).