Copy pages starting with prefix for every language. Add lang to context.

gatsby-node.js

import { getLanguageFromPath } from './utils'

exports.onCreatePage = ({ page, actions }, pluginOptions) => {
  const languages = pluginOptions.languages
  if (!languages) {
    throw Error(
      `Plugin "gatsby-plugin-i18n-pages" require languages`
    )
  }
  const { createPage, deletePage } = actions
  return new Promise(resolve => {
    if (!page.context.i18nPage) {
      if (page.path.startsWith(pluginOptions.prefix)) {
        // create page for each language
        const prefix = pluginOptions.prefix
        languages.forEach(language => {
          const newPage = Object.assign({}, page)
          newPage.path = page.path.replace(prefix, `/${language}/`)
          newPage.context = {
            ...newPage.context,
            lang: language,
            i18nPage: true,
          }
          createPage(newPage)
        })
        deletePage(page)
      } else {
        const newPage = Object.assign({}, page)
        newPage.context = {
          ...page.context,
          i18nPage: true,
          lang: getLanguageFromPath(page.path, languages) || languages[0],
        }
        deletePage(page)
        createPage(newPage)
      }
    }
    resolve()
  })
}