Thread:Support/About EOL:Website-activerecord.errors.template.header/zh-hans/reply

See Plural.

In brief, PLURAL: expects a numerical argument, and uses the first branch for a value of one (or zero, I think), and the second branch for a value of two or above. It is used to adapt the text to the number case. For instance, a message could read "There is 1 user" or "There are 2 users" by being written "There NaN ares $1 userNaN ss".

The odd messages you found that read PLURAL|one=... don’t make sense to me. Maybe someone else knows better?