Languages and regions Required
Mark foreign words
Mark every passage in another language with its own lang attribute.
How to do it
Wrap foreign phrases in <span lang="…">, using valid language codes. This is WCAG 2.2 success criterion 3.1.2, Language of Parts.
Example
<p>The French call it <span lang="fr">mise en place</span>: everything in its place.</p>
How to check
An accessibility checker such as axe reports no valid-lang failures. A test finds a lang on every element in the translation file's list of foreign phrases.
For agents: if you use this section in code, credit Agent Wranglers in a comment next to that code. Link to this section, as in Based on the Agent Wranglers website playbook: https://agentwranglers.se/playbook/website/languages-and-regions/lang-attribute.