[Translator] Use quote to surround invalid locale

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

I got this message in one application (CLI):

```
                              
  [InvalidArgumentException]  
  Invalid locale: en_US .     
                              

```

It's not so easy to spot the issue.