West Frisian

West Frisian is the language that was spoken in the Dutch province of Friesland, also known as Fryslân in West Frisian. West Frisian was recognised as an official language by the Dutch government, and schools in Friesland were mandated to offer classes. As a result, control of the language was rising in the decades before the downfall.
Initially, the downfall did not impact this. As the sea levels kept rising, Friesland started getting flooded, large parts of it slowly becoming Drowned Lands. Over time, a large part of the population had to flee to other provinces, but even then they still spoke Frisian and taught it to their children. There also were still several settlements on Frisian soil, where Frisian was proudly spoken.
However, this changed as settlements became more paranoid over time, due to the increasing scarcity of technology and rising conflicts. Hostile treatment befell anyone speaking a language many others did not understand, out of fear they were keeping secrets and planning betrayal. Many Frisians stopped teaching their children, out of fear of prosecution. Others proudly refused, which in many cases led to violence, sometimes even deaths.
Meanwhile, some of the remaining Frisian settlements fell to raiders, while others were destroyed by betrayal, such as Aldterp. The one remaining settlement was Zwartterp, where the Elders refused to teach the younger generations any West Frisian. Instead, the children learned English, with a few Dutch words sprinkled in. The reason for this was that the Elders wanted to keep their secrets, and speaking their own language served this purpose well. The side-effect was that they forced their own language to die out over time.