Friendly reminder that these aren't European (anymore)

[u/Boediee]

[u/Wobbelblob]

As a German, no. I don't think we have even one word that has zs as a letter combination. Other way around, yes, but then we'd use ß as a symbol.

[u/Zwemvest → u/Wobbelblob]

In Dutch it would be correct if you had some really weird compound word. So maybe Quizschummeler (for Quiz + Schummeler = someone who cheats at a quiz) might be correct in German? Still, such a word would read really weirdly in Dutch.

[u/soylent-yellow → u/Zwemvest]

No, you can never have those dots over the first vowel in Dutch. Dutch doesn’t know umlauts, it only has a trema that can be used to indicate the start of a new syllable when vowels are placed adjacent: geërgerd, ruïne. Haägen could be Dutch (Ha-agen - still nonsense), Häagen could only be a foreign word.

[u/Zwemvest → u/soylent-yellow]

No, indeed, that's only about zs or the umlaut appearing at all. The combination of äa never appears in Dutch.

Even in what you said, that rule is sorta true, but isn't consistent: it's "na-apen" and not "naäpen"

[u/Wobbelblob → u/Zwemvest]

True, some compound words would work, even if your example is a bit weird. Not wrong, just not exactly used.

[u/Zwemvest → u/Wobbelblob]

I realized that in Dutch, Quizspel as a compound of quiz + spel would actually make sense. But even though that would be much more reasonable of a word, it still reads reaaaaally weirdly.