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.