Allow i18n to have only one locale

[martinjagodic]

Is your feature request related to a problem? Please describe.
If you configure this either globally or on a collection

i18n:
  locales: [en]

you get this error: 'i18n.locales' must NOT have fewer than 2 items

If you want to temporarily disable a locale or you are testing only one, this is quite annoying. You would have to change the structure of the i18n object every time, which can be time-consuming if you have a lot of those on file collections for instance.

Describe the solution you'd like
Allow i18n.locales to have only one item

Describe alternatives you've considered
Currently, you need to set i18n: false, which is not technically the same.

[dfdez]

Following this issue I'be been working in some use cases in which is useful to be able to have only one locale.

As @martinjagodic you could set the config to i18n: false, but technically is not the same. Mainly because the final file / folder structure is going to be completely different.