While you're correct, it is historically a loanword from Portuguese, not from Spanish. You can verify this in your favorite 国語辞典 if you want to. Like many loanwords, the pronunciation got corrupted a bit. (An example in the other direction: Godzilla, which you'd have to take several stabs at before a Japanese person guessed you meant gojira.)

sorry, I don't have japanese installed. What's the romaji for the word you used after "favorite"?

kokugo jiten -- a Japanese language dictionary

Ão is a notoriously difficult diphthong to pronounce for many native speakers of other languages. Wikipedia says that it was even used as a shibboleth in the Paraguayan War, but it doesn't cite sources.

And Japan in portuguese = Japão.

japonês?

which sounds just enough like "pahng" that a non-portuguese, learning about bread might turn it into "pang" or "bbang" or "bao" depending.