Make Translation Languages Configurable

[sshaw]

Currently the list of translations is hardcoded. A user should be able to configure the languages they want translated; both source and destination languages, via the UI.

Currently we're using the iso gem to validate but I don't think validation makes sense since there is no true set of valid language codes. For example, ru-US is not used but valid.

As for what spec to use, I don't know if it's important, or at least it should not be exposed via the UI. Maybe it just provides Language and an option Country and we use whatever format we want internally.

If we do expose the code and then see it necessary to use language variants, e.g., gsw-u-sd-chzh for Zürich German, this can just be confusing for a user.

If there's a need for the ISO gem, we should look into how it compares to this: https://github.com/xwmx/iso-639