The Netherlands should be highlighted for ë but isn't

They've split Finland pretty arbitrarily into areas where (supposedly) Swedish speakers are found for 'å', but there's really no reason for it. The letter is a part of the Finnish alphabet and taught to everyone in school, so it should cover the whole country I think.

Ireland should be highlighted for Éé

Swedish uses é

All Polish letters are included. But I don't understand, why a small piece of the ocean is marked along with Poland in "Ż".

Edit: I checked, it's Malta.

English definitely uses æ even if rare

Are you viewing this version? Sometimes preview links get funky

https://files.catbox.moe/4t2rzv.png

The Griko people in southern Italy use Latin alphabet though.

æ is in purely Latin words like ex æquo, et cætera, or curriculum vitæ, that's all. œ appears in œil (eye) so you see that a lot more commonly already, but I can't think of any other word that uses it off the top of my head (beside other derivated words like œillères). (pardon the puns)

Wikipedia gives examples of "curriculum vitæ" and "et cætera." We use those both as loanwords in English, but I've only seen it as the separate letters "ae," not the ligature æ.

The Nordic languages use ö or ø instead, in Swedish also ä is used instead of æ.

It's so similar it's basically two accents of one language...

According to German Wikipedia it is old spelling and thus, no wonder you didn't come across it:

In Italian, the circumflex used to be used primarily in the pluralisation of words with a final -io to mark the coincidence of two -ii: il principio "the principle" → i principî, in contrast to i principi, the plural of il principe "the prince". In addition to principî, there was also the full spelling principii, which was not pronounced correctly. Today, the words for "principles" and "princes" are spelt principi without distinction.

(translated using DeepL)

According to the English article, it is also used in Emilian and Friulian. In both, a long vovel is indicated with a circumflex.

That's just the encoding, you want to thank unicode.