préférer l'inversion "Comment se fait-il que..."

Je ne suis pas trop d'accord. La phrase en japonais a un ton moins formel ...

En français, l'inversion dans une question n'a rien de formel. C'est juste du français correct.
"Comment ça se fait'" est du language parlé incorrect.

par ailleurs, en français, on met une espace avant les points d'interrogations et d'exclamation.

Voila, c'est changé mais çà correspond moins au ton japonais ...

Votre perception sur la question est erronée. Je vous assure, en tant que francophone natif depuis 48 ans, qu'il n'y a aucune différence de 'ton' entre "Comment ça se fait" et "Comment se fait-il". Il y a juste une différence de français. La première est incorrecte mais commune, je vous l'accorde, et l'autre est correcte et également commune.

You gave Paul a hard time when he said he was a native speaker, ignored two other native speakers, and ultimately did not change your sentence (not even with Trang's convincing Google argument). And now you use the native speaker argument here, when it's in your favor.

Either respect nativity or don't, but please pick one.

Ah, Super-Linguist, the planet's hero, coming to the rescue of his dog!
I was thinking of you shopping in my local grocery, because they advertise bio products with collectibles for children to make them become "heroes of the planet".
To come back to your revenge, defining "native" is not that straightforward: I am 48, Paul is 40. I have been learning English since I'm 5, have British family, lived and worked in the UK and North America, had 2 british companions and a canadian one, and spoke and read far more English than French over the last 25 years as a result.
I don't know about Paul's record and this is none of my business, but we're not that far way on English...I'm certainly closer to native in English as he is in Japanese in any case. I used to teach different subjects to Brits and I corrected their English more than once...
Anyway, kurisutofu has a wrong perception of the inversion of questions in French (which is the standard way of putting a question in that language...) and that's that. Check with any expert in French.