"Related" heading not changing when site language is changed

In both cases where this has been observed, the user had their site set to French with Related Posts enabled. They changed the site language to English but the heading remained "Sur le même thème" instead of changing to the expected "Realted" in English.

#### Steps to reproduce the issue

1. Disable Related Posts
2. Set site language to French
3. Enable Related Posts
4. Set site language to English

#### What I expected
When the site language is set to English, I would expect the heading "Sur le même thème" to change to "Related"

#### What happened instead
Instead it stayed as "Sur le même thème". To fix this, I disabled the Related Posts feature, then re-enabled it and that set the heading to "Related" finally.

User reports: #13164137-hc and #13250511-hc

Start by reproducing the listed language-change steps with WordPress and Related Posts enabled, then trace how the Related heading is stored and refreshed when the site language changes. Done means changing French to English updates “Sur le même thème” to “Related” without disabling and re-enabling the feature.

Written by the indexing model from the issue text.