Well, my 3 most recent languages-in-process (Siidmak, Merahai, and an
unnamed experiment) haven't yet developed any cultures at all. Heck,
they've barely developed a usable amount of vocabulary.

The speakers of one of my earlier languages, ea-luna, have a state
religion worshipping a creator Goddess [Iludina] that they feel has
chosen to dwell among them (as the spirit of the land they live on).
They (ea-diwe, The People) also have clan gods, family gods, and
household gods, as well as gods and spirits that are associated with
certain locations.

Ea-diwe aren't the most religiously tolerant people on their home world,
and have been known to execute linguists for suggesting that their
language may have developed from a previous languages (or even might be
related to neighboring languages), since *everyone knows that ea-luna is
the language of Iludina, and is a special gift from her to her people*.

As I mentioned with regard to numerology, the speakers of my very first
language had seven god/esse/s that had particular associations and
aspects, but I didn't really develop the mythology too much.