Thanks to the down-voting, I was able to notice it, there isn't such thing as "mens" or "mens'"! I only meant to say that "men's coffee" is right, and of course I was wrong, jaye16's comment brought me back to reality! That would have been more like "ανδρικός καφές" or "καφές για άνδρες".
Η σοκολάτα pluralizes as one might expect: οι σοκολάτες (like η φούστα → οι φούστες, η τσάντα → οι τσάντες, etc.). Ο καφές pluralizes to οι καφέδες.  Other words that fit this pattern are described by D_.. below.
Loanwords that don't "fit the Greek cadence" tend to not decline: the plural of το πάρτι is τα πάρτι; the plural of το τσίζκεικ is τα τσίζκεικ, etc.
There are many more nouns that do not have the same number of syllables in plural.
Lots of them are
masculine ending in -ες like καφές (κεφτές, μενεξές, μπαξές all from a -έδες plural),
then some masculine nouns ending in -ης like μανάβης-μανάβηδες, τελάλης-τελάληδες,
and then there are all the neuter ones mostly ending in -μα, like όνομα- ονόματα, αίμα-αίματα, πτώμα-πτώματα, πάτωμα-πατώματα, δόγμα-δόγματα
but also others like δόρυ-δόρατα.
Some feminine words as well: μαϊμού-μαϊμούδες.
In short, it's not unusual for nouns to change in the plural, they are just taught as 'uneven-syllable nouns'.
There is a difference between: "Men's coffee" would mean the kind of coffee men drink. Or another example: "Men's coats button from the left to the right but women's coats button from the right to the left." Whereas, "the men's coffee" means the coffee which belongs to the men.