How does one say
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We do use the expression "macho alfa" in Spanish (the greek letter alpha translates to alfa in Spanish.)
I haven't heard much, if at all, the expression "hembra alfa" but that would be the translation, and we can see in the ngram viewer that it's somewhat used.
Other alternative would be to use "dominante" instead of "alfa", although there we tend to use "hombre" and "mujer" instead of "macho" and "hembra".
The ngram graph for "hombre dominante" and "mujer dominante" is priceless.