Make translatable

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Currently all strings are hardcoded in English. Would be great if we could make them localizable somehow.

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https://medium.com/stencil-tricks/implementing-internationalisation-i18n-with-stencil-5e6559554117 seems to be a sensible starting point, but:

• fetching the JSON seems to assume an installation in /
• it's unclear if this will work with the development server

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These posts address the problems above:

• https://dockyard.com/blog/2019/04/16/lazy-loading-custom-assets-with-stenciljs-part-1
• https://dockyard.com/blog/2019/05/23/stenciljs-lazy-loading-part-2-npm-build-assets