birbday

I’ll never understand how a suffix like -eaux even exists, and essentially melts down to -oh.

It's actually worse. It's two suffixes. With one of them typically having no phonetic realisation.

The -eau suffix used to be pronounced [ɛ̯au̯] → [e̯au̯] in Middle French, that's why it's spelled this way. Then the [au̯] got simplified into [o], and the [e̯] got lost; but by then the orthography was already standardised, so it got stuck this way.

That isn't even the weirdest part of it. That -eau is cognate to Italian -ello. Nowadays they neither look nor sound even remotely the same.

Then there's that -x. It's a plural mark. Originally it wasn't even -x, it was a ligature for Latin -us that kind of resembled a ⟨x⟩. Languages hate this sort of "similar, but different" situation — if it looks like ⟨x⟩ it gets treated as ⟨x⟩, so the ending -s for what used to be /s/→/z/ got spelled as -x.

Then French got rid of all syllable-final /s/ and /z/; you see this for example in the name "Descartes" being pronounced as /de.kaʁt/, note how there's no /s/. That applied even to plural /z/, that became Ø...

...well, in most situations. See, plural -s in Romance languages often piggybacks the next word, if said word starts with a vowel, becoming the onset of its first syllable. But then it wasn't a syllable-final /z/ any more, so it avoided deletion. So that ⟨x⟩ is still haunting people around, both to highlight "hey, this is a plural!" and because sometimes it's actually pronounced.

Danish
Hvor er H og R i hvordan? ("Where is H and R in the Danish word for how?")

Norwegian borrows a lot from Danish, but at least Norwegian cares more about intonation to separate words, almost sounding like a gallop. "<.<

French Comparing to Portuguese and Latin, some stuff makes sense, but still kinda of a stretch. Curious, I think, specially as France doesn't have, to my knowledge, as many natural formations as other countries to isolate its people into diverging the language.

Sure you can understand it - it's quite simple, and only requires two sound changes (though the reality of the vowel fusion was probably a bit more complex).

All of the sounds in -eaux used to be pronounced. (Three vowels pronounced together like this (without hiatus) is called a "triphthong", if you're interested.) But, as we famously know from other words in French, final consonants were lost (under specific circumstances), so -x stopped being pronounced as part of that general change.

Also, -eau underwent fusion. The three vowels sounds coalesced into a single sound that preserved parts of all of the three original vowels. The height of the [e] was preserved as a mid vowel, the backness of [a] and [u] were preserved, and the rounding of [u] was preserved, resulting in [o].

Just like English, however, French orthography wasn't updated to account for this change, and so we can see the history of the language in the differences between the way it's written and the way it's pronounced.