Your question is really interesting, in fact, some "language lessons" that promises teaching portuguese speakers learn English, saying like: "Learn lots of English words in 5 minutes", they tell the students that Portuguese Words ending in -ão change to English in -ion. But it is not simple though.
English has its roots in some Languages, according Wikipedia, about of 30% of English words were influenced by French. I read a time ago, words ending in -ion were influenced by French. But as I said, not of them are translated to Portuguese in -ão.
Lets compare some English words ending in -ion and their translations to French and Language:
Transformation -> Transformation -> Transformação
Formation -> Formation -> Formação
Action -> Action -> Ação
Abortion -> Avortement -> Aborto
Transportation -> Transport -> Transporte
The word congestion can be translated to Portugese in Congestão and has its French word congestion.
I can't tell you its a rule, but I think the words ending in -ion whose French correspondence is the same, are translated to Portuguese as ending in -ão.
But the others ones not ending in -ion in French are not translated to Portuguese in -ão