Learn Spanish (Spain): Hoy trabajan aquí noventa hombres.

No. Unlike the number uno or the hundreds (like doscientos), the tens from 30 to 90 never change their endings for masculine or feminine words. It is always noventa, whether you are talking about noventa hombres (ninety men) or noventa mujeres (ninety women).

How can I easily remember that "noventa" means ninety?

It comes from the number nueve (nine). Just like in English where "nine" becomes "ninety", in Spanish the stem changes slightly to nov- and adds the -enta ending, which is the standard ending for almost all the tens.

The English translation says "are working", but isn't "trabajan" just the simple "they work"?

Yes, but in Spanish, the simple present tense (like trabajan) is frequently used to talk about actions happening right now. Therefore, it can translate to either "they work" or "they are working" depending on what makes the most sense in the English sentence.

Why is the word order flipped to "work here ninety men" instead of "ninety men work here"?

Spanish word order is much more flexible than English. When a sentence starts with words expressing time or place, like Hoy (today) or aquí (here), it is very common to put the verb (trabajan) before the subject (noventa hombres). You could also say Hoy noventa hombres trabajan aquí, but the flipped order used here sounds perfectly natural to Spanish ears.