It's the short form of {{lang|ja|}}, which is the Japanese part of {{lang}}.
{{lang|ja|}}
{{lj|CONTENT}}
It is not recommended to use this template in forms and other block-level elements, because this template is an inline element.
{{lj|初めての恋が終わる時}}→初めての恋が終わる時
{{lj|初めての恋が終わる時}}
