Why is Ansible described as "the language of DevOps"?

Because a playbook is readable by everyone along the path from development to production, so dev/test, QA, operations, management and outsourcers all work from the same document.

* Every role from development to production reads and runs the same playbook. *

A playbook is a numbered list of steps in plain YAML. That makes it a shared artefact:

• Dev/test writes or reads it to set up their environments.
• QA uses the same playbook, so test systems match production.
• Operations runs it in production.
• Management can read what will be done without knowing a programming language.
• Outsourcers (external partners) get an exact, executable description instead of a vague handover document.

The same playbook travels from development to production, so "it worked on my machine" problems shrink: every stage was built by the same instructions.

Go deeper:

• What is an Ansible Playbook? (Red Hat) — why a YAML playbook works as a shared, reusable plan across teams.