Here is what I found online in answer to the question why ihmnen puhuu is in 3rd person singular when the meaning is plural:
In formal Finnish, moni is the plural marker and the following noun (if any) and verb are in the singular.
In informal Finnish the plural form "monet" is used and the modified noun and the following verb are plural.
moni lapsi syö puuroa aamiaiseksi (formal language style) many children eat porridge for breakfast = many a child eats porridge for breakfast monet lapset syövät puuroa aamiaiseksi (informal language style) many children eat porridge for breakfast
This is confusing to me. Fred Karlsson's FINNISH AN ESSENTIAL GRAMMAR gives the formulation "Monet ihmiset ajattelevat paremmin, kun he ovat juoneet kahvia". (Many people think better when they have had some coffee). The Duo finnish "Moni ihminen puhuu..." here is strange to me.
The Wiktionary article explains that the singular form is used in more formal Finnish. The DL course uses this singular version because we haven’t yet learned the plural forms. No need to be confused about it, just keep in mind that many languages have different ways to express concepts in a formal vs. an informal way.
And ultimately, we need to be familiar with both ways.
Agreed. I believe both of Hacu's sentences would translate to "many people speak Spanish". It's just that in Finnish, "moni" goes with a singular noun and so the verb takes a singular ending.
The grammar notes suggest that if this is confusing, to imagine it as "many a person speaks", or whatever.