Translation:Morality is the solid foundation of man.
It should be "human" or "humanity" instead of "man", because "man" is usually used to refer to a specific human and usually male.
Nah, "man" is just a more archaic/older-fashioned way of saying "human" or "humanity" depending on the situation. "Man" is of Germanic origin while "human" and "humanity" are from Latin via French.
"Men" carries a stronger association with males than "man" (which oftentimes is used poetically to refer to humans).
Sometimes people get upset over misunderstandings like how the word "man" in spokesman just means a person or human but they've come up with spokeswoman to "avoid sexism".
That's why I said "usually". As the word is archaic, it is used less at the present.
It's still used a fair bit in biology papers, religious and philosophical works. As the sentence in question is one that relates to morality, using "man" is perfectly acceptable and I'd say a tad better sounding.
If it was thought necessary to add the prefix wif (wife) to man (human being) to form a word designating a female person exclusively, then maybe, just maybe, man never was a "neutral" word to begin with.
Languages are not static as meanings and associations evolve and shift over time. It's generally accepted that the word "man/mann" could originally refer to humans as a whole and only later on was used largely for the meaning of a male as other terms for it died out.
Other common terms for a male existed such as guma & wer which only exist in Modern English in compounds like bridegroom (though it can be shortened to just groom) & werewolf. There were also multiple words for a female such as wīf and frea.
Over time, the word wīf (which originally just meant woman, female) developed the compound wīfman(n) and the meanings diverged wherein the original wīf developed into a "married female" while wīfman(n) assumed the role of a female/woman. The compound woman (wīfman(n)) itself just means "female person".