[RPG] root language from which multiple Forgotten Realms languages were derived

You've found them all.

The Forgotten Realms was first published for AD&D 1st edition. When 2nd edition came out, the change in spells and available classes was explained through the Godswar aka The Time of Troubles.

The introduction of D&D 3e was exceptional in that there was no overarching, global in-setting event introduced to explain the rules changes. Oddly, the adventure Die Vecna Die! was intended to explain the rules changes (for all D&D settings), but its events were never made FR canon. WotC just proceeded with the 3e rules and setting changes without adding any corresponding historical events. The 3e change is thus “silent” in FR's historical record.

Karsus's Folly was made global setting canon with 3e as a historical detail that further developed the history of Netheril and explained some changes local to the Anauroch and some new game elements, but it wasn't the same kind of global upheaval—it was more of a retcon slipped in as undisruptively as possible.¹

The Spellplague (and about a century) explained the shift from D&D 3.x to 4e, and the Second Sundering did the same for the transition to 5e.

1. ^{Although Karsus's Folly was first described in a 2e adventure, the way 2e setting publishing worked meant that only the frozen-moment-in-time documented by the basic setting boxed set was canon. DMs were given explicit authority over what was canon beyond that. Thus changes made by adventures were only canon in a campaign if a DM used those in their campaign — because of course, the PCs might change the outcome! D&D 3e cemented Karsus's Folly as the official way it happened in Realms history for that revision of the base setting, so though it took place earlier, it wasn't canonised until 3e.}

The Feast of the Moon seems the closest. A summary from the Forgotten Realms wiki article:

The Feast of the Moon [...] was an annual festival occurring between the final night of Uktar and the first day of Nightal -- traditionally marking the onset of winter. It was also a time to celebrate and honor the ancestors and the respected dead. Folk blessed their ancestors' graves and performed the Ritual of Remembrance. People also gathered to tell stories of the deeds of their ancestors and of the gods until deep into the night, until they merged and become legend. This was a time to hear of past heroes, great treasures, and lost cities.

In Faerûn, battles were typically fought between harvest-time and the coming of winter. This meant that most of the fighting usually occurred in the month of Uktar. The timing of the Feast of the Moon—after recently slain soldiers had joined the ranks of the dead—was thus practical, if sadly ironic.