Stubbing out I18n lookup

Prior to these measures, the translation services were taking 0.4
additional seconds to run the tests. This was growing over time.

```console
$ rspec ./spec/controllers/sip/citations_controller_spec.rb:22

Finished in 0.09408 seconds
```

```console
$ WITH_I18N_DICTIONARY=t \
  rspec ./spec/controllers/sip/citations_controller_spec.rb:22

Finished in 0.48515 seconds
```