don't translate error messages twice

As @stof pointed out in #12164, error messages are already translated
when they are passed to the template.
bug #12322 don't translate error messages twice (xabbuh)

This PR was merged into the 2.6-dev branch.

Discussion
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don't translate error messages twice

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

As @stof pointed out in #12164, error messages are already translated
when they are passed to the template.

Commits
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4bd8661 don't translate error messages twice