In general, you use the singular with "many a":
many a man came to try his luck, but none could untangle the knot.
The problem is that when you use "many a" you need to treat it as a collection of individual items, not as a group. For example, neither of the two following sentences work:
*Many a man can carry only a small stone, but working together, he could move a mountain.
*Many a man can carry only a small stone, but working together, they could move a mountain.
You(although the second is not as bad). You have to drop the "many a ..." construction to get this sentence to work:
Many men can carry only a small stone, but working together, they could move a mountain.
You're treating "many a language" as a group in thisyour sentence.