Integrate gatsby-theme-i18n and react library

[pmelab]

• add gatsby-theme-i18n to the @amazeelabs/gatsby-starter or @amazeelabs/gatsby-theme-core
• choose a react translation library and integrate it too
• contribute patch to gatsby-theme-i18n that allows to enforce languages prefix for default language

[pmelab]

The default prefix has been resolved: gatsbyjs/themes#75

[pmelab]

We mostly used i18next. In a recent prototype I used react-intl which turned out to be less feature-rich, but more straightforward. We are more on the CMS/Website end of the spectrum and don't have a lot of need for App-centric features. We should do a more detailed investigation on what fits our needs:

• https://www.i18next.com/
• https://formatjs.io/docs/getting-started/installation/ (https://github.com/alibaba/react-intl-universal)
• https://lingui.js.org/

[pmelab]

We decided to go with react-intl and not using gatsby-theme-i18n since it interferes with pages that are automatically created from the CMS. Directly controlling creation of pages is more straightforward.