Issue #3618523: Make the translation check advisory, like the other validate jobs

Follow-up to !277 (merged), on the same issue.

The translations job becomes allow_failure: true, at the same level as cspell and phpstan.

A string waiting for its French is not a broken build. Refusing a merge over one punishes the wrong thing, and it puts a documentation-shaped finding on the same footing as a failing test, which nothing else in the validate stage does.

What makes an advisory job trustworthy here is that the pipeline is read job by job rather than by its badge, which is the only way to see any of the validate stage: every one of those jobs is allow_failure, so a red one never shows on the badge.

The comment above the job and the section in docs/translation.md say the same thing, so nobody has to infer the job's weight from its YAML: it reports, and the script is one second locally, which is where a missing string is cheapest to find.