Localize the onboarding wizard

[atimmer]

This makes it possible to translate the next and previous buttons

Inspired by i18n-calypso, but Automattic/i18n-calypso#19 blocks us from using it.

Required for Yoast/wordpress-seo#5582

[yoastbot]

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[jcomack]

Please use double quotes.

[jcomack]

CR done. 👷

Acceptance done as well. Translations work, however I do get the following error.

I cannot get rid of this issue despite clearing out my node_modules and reinstalling. Also, for some reason, I cannot see why the Travis builds keep failing, as Travis says the repository doesn't exist.

[atimmer]

The error has nothing to do with this PR. I have fixed it on develop when not using npm link.