Late controller flashes translation

| Q                    | A
| -------------------- | ---
| Bug fix?             | no
| New feature?         | no
| BC breaks?           | no
| CI currently passed? | no
| Tests pass?          | yes
| Scenarios pass?      | yes
| Checkstyle issues?   | no
| Changelog updated?   | no
| Fixed tickets        |
| Doc PR               |

AbstractController could be lighten by removing the dependency on translator, but it would break BC.
This is the subscriber priority.
As it must be executed at the very last (because it transforms Flash objects into string), its priority is very low (-128 is an arbitrary number meaning that 128 (0 => 127) subscribers can be ordered before this one).