Add guidelines around avoiding translation updates

We have been losing some of our translation progress due to un-nessecary
translation updates. Add some guidelines around avoiding this so that we
don't create duplicated effort for our translators.

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/245431
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/47078

doc/development/i18n/externalization.md

@@ -480,6 +480,21 @@ This makes use of [`Intl.DateTimeFormat`](https://developer.mozilla.org/en-US/do
 
 ## Best practices
 
+### Minimize translation updates
+
+Updates can result in the loss of the translations for this string. To minimize risks,
+avoid changes to strings, unless they:
+
+- Add value to the user.
+- Include extra context for translators.
+
+For example, we should avoid changes like this:
+
+```diff
+- _('Number of things: %{count}') % { count: 10 }
++ n_('Number of things: %d', 10)
+```
+
 ### Keep translations dynamic
 
 There are cases when it makes sense to keep translations together within an array or a hash.