Which English diphthongs are you talking about?

There's the /ɔɪ/ in "choice", and /aʊ/ in "mouth".

There's /aɪ/ in "light".

Sometimes you hear a distinct /ʊ/ in "slow", but again meaning there's an /oʊ/ diphthong. But, if you dropped that sound and just pronounced it /o/, I think nobody would notice. I think you could argue that many dialects of English do that already.

There's also a claim that /eɪ/ is heard in words like "play", and I can maybe see that, but there's also a claim that it's how you pronounce "face", and everyone I know just uses /e/.

There's /juː/ as used in words like "music". But, many of the words where that one was once used just use a /uː/ like "student" or "tune".

So, that's only /juː/, /ɔɪ/, /aɪ/ and /aʊ/ where the word sounds completely wrong if you don't use the diphthong, and a number of other cases where some dialiects use a diphthong, or some people claim to hear a diphthong.

Meanwhiile in Portuguese you have:

• /aj/ - pai
• /ɐj/ - leite
• /ej/ - rei
• /oj/ - dois
• /uj/ - fui
• /aw/ - mau / mal
• /ɐw/ - saudade
• /ew/ - seu
• /iw/ - viu

And then add to that all the nasal diphthongs like mão or não. There are a lot of other weird things going on with Portuguese, but really the only English dialect(s) with a similar amount of vowel sounds is Aussie / Kiwi English.