i18n: support internationalization

[jakedoublev]

We should begin to think about internationalization of things like:

1. long manual help output
2. flag help output
3. error messages

If we unified errors into codes first like ngrok, we could apply translation more easily. As a first step, providing translations for each error code within distinct documentation is a lower effort solution. A future step could be translation based on a language configured within a profile or environment variable.

[jrschumacher]

@jakedoublev could you post some of those findings on the tooling.

[jakedoublev]

Of course. Some interesting options:

• Go i18n: https://github.com/nicksnyder/go-i18n
• Markdown i18n: https://github.com/ultrabug/mkdocs-static-i18n
• Contributing to Cobra directly as there is slow and steady traction on an internationalization issue for i18n within CLI help text: Multi-language support for CLIs? spf13/cobra#1134