You can call this operation "setting the plurality of" the word. This implies you are determining whether to make the word singular or plural. ("to set the plurality of", "set the plurality of", "the plurality was/will be set")

One can call this operation "setting the plurality of" the word. This implies you are determining whether to make the word singular or plural. ("to set the plurality of", "set the plurality of", "the plurality was/will be set")

In this use case, "SetPlurality" should make it clear to observers what the function does. As a programmer i would think of it as "set plurality to true or false".