I have a web site with mainroad theme. In the contents folder there are three subfolders: about, citaten and opinie. The latter two have an _index.md and subdocuments. In hugo.yaml there is

sectionPagesMenu: main

The names of the sections appear in the main menu, however, they are pluralized. citaten becomes citatens and opinie becomes opinies. This may or may not be desired, but it is wrong since it bypasses i18n.