German and English pluralisation is quite different, but since it’s a feudal country, I’d assume it could use currency conventions from Feudal Europe: Mostly, the Latin Denarius (pl. Denarii) and the German Pfennig (pl. Pfennige).
I would, however, suggest just combining all your options:
A single Tahlin.
A countable amount (How many Tahlen do we have),
An innumerate quantity (How much Tahler does it cost).
I like the idea of keeping it Tahlin for singular and plural, with the only way to tell is through word usage/sentence structure, and context.
A Tahlin came knocking on the door and wanted to sell me a newspaper subscription.
An angry horde of Tahlin were threatening to overthrow the capital.
This way you can have fun writing sentences where the player does not know if it’s singular or plural, maybe because one character is trying to talk to another character in secret.