Adding a REL to wordpress header

Hello I am having problems with putting in a Rel statement for our website.
http://hamilton-seventh-day-adventist-church.com/
I think I know what to put in but not sure how to put in or where to go to do it.
I think it needs to go in the header for the website. This is an example what I am trying to put in. This seems to refer to my attempts to resolve my language issues.
https://ww.wp.xz.cn/plugins/language-selector-related/
The problem is I am not sure what to put, or where to do it in WordPress.