Should I learn a database language before backend, or vice versa? I’m thinking that I learning Rails and some form of SQL.

Rails has its own way to interact with the database (active record) which don't need sql, but knowing sql will help a lot.

You can also use a npswl database which has it's own way of interarction with you, not realy a query language.

Also, rails is jjst a framework, you should already know ruby before going to rails.

So what I suggest is:
Learn ruby
Learn databases
Interact with databases using ruby without rails.
Learn rails.

Do not l....i repeat.. DO NOT neglect relational db knowledge.

Regardless of all the ORMs out there stick to learning raw SQL before venturing into any sort of orm.

Truuuuuuuuust me.