I've seen in my language the word "millonarismo" being mentioned several times but with quotations marks. Then I had the idea of using the translator to input "millonarism" to see if it translated into something in spanish and it doubtfully translated it into "millonarismo". I looked for examples of the use "millonarism" in English and I couldnt find any. Now I have both doubts, my original doubt in spanish and this other one, is "millonarism" a valid English word? or the english translator is taking it as valid input wrongly? Note: the translator suggests "did you mean millenarism?"
Yes - According to Merriam-Webster millionairism is actually a word, but it's not a word that's ever likely to be used in conversation. English speakers will use different words instead, like: elite, upper class, the one percent, the rich, the powerful, posh, etc.
"Millionarism" (or actually "millionairism") is a word in the dictionary, but it's not in common use in the way you describe. Still, that doesn't mean it can't become common. People make up new words all the time, or use old words in new and unusual ways, or combine two words into some new word that's a combination of the two, often with additional nuance.
For example "Trumpism" is a word that variously means "the words and policies of Donald Trump" or "an outrageous lie told with sincerity, akin to numerous statements by Donald Trump". Trumpism didn't really exist before 2016 (and possibly not even before 2017) but now it's inescapable.
You have yet to define what millionarism means, though, so it's probable there's already a word in English for this concept. For example, if it means "the kind of attitude or lifestyle that makes someone a millionaire" then possibilities are:
(the) millionaire mentality
(the) millionaire mindset