Tydotsu (pronounced sort of kind of like tea-DAWT-soo, IPA /ty.ˈdɔt̚.ʦu/) is a country in Sasaro, to the north of Súiacúil and to the south of Lwait. It's mainly the land surrounding the River Tydotsu, plus some additional land around the ocean coast and some islands in the ocean. The country is inhabited by both humans and mermaids (living in the river); there are separate governments for humans and mermaids, plus a government that's responsible for human–mermaid interactions and humans swimming in the river. 'o̬o̬rûîi (pronounced something like oh-roo-ee, IPA /ʔoː˩.ɻuiːː˥˧/) is the mermaids' name for the river.
Most of the country is forest.
The Tydotsuy people were at one point fairly far upriver (west), near Vegúil, and there were many conflicts between the two groups of people. The Tydotsuy people went east along the river in search of land to call their home, and then continued expanding to the ocean. Along the way, they encountered the mermaids and came to the agreement that they currently have.
At that point, most humans who were citizens of Tydotsu were all of the same race, identifiable by their blue eyes (TODO: other features?). Later, they started allowing more freedom of movement into and out of their country; the countries around it already allowed such freedom.
There are two main languages spoken in Tydotsu: Tydotsuy (literally "from Tydotsu") and Li̬o̬a 'e Gweû (literally "language of mermaid"), the former being spoken by humans and the latter by mermaids. Li̬o̬a 'e Gweû is a somewhat common second language for Tydotsuy humans; mermaids learn Tydotsuy less often because they have a harder time pronouncing it. Tydotsuy has a native writing system that was developed fairly late, and is therefore mostly phonemic. Mermaids don't use writing much, but humans have developed a writing system for them based on the Tydotsuy writing system.
Tydotsu's national colors are blue, representing water, and green, representing plants. Its national symbol is a shape made of concentric circles, representing the shape a drop makes when it hits water. Its motto is Nujoboosnut waase olyy húruphuh fuþjo'hoy ("The water of the river gives us life").