gatsby-node.js

const _ = require('lodash')
const path = require('path')
const { createFilePath } = require('gatsby-source-filesystem')
const { fmImagesToRelative } = require('gatsby-remark-relative-images')
const locales = require('./src/constants/locales')

exports.createPages = ({ actions, graphql }) => {
  const { createPage } = actions

  return graphql(`
    {
      allMarkdownRemark(limit: 1000) {
        edges {
          node {
            id
            fields {
              slug
            }
            frontmatter {
              templateKey
              locale
            }
          }
        }
      }
    }
  `).then(result => {
    if (result.errors) {
      result.errors.forEach(e => console.error(e.toString()))
      return Promise.reject(result.errors)
    }
    const posts = result.data.allMarkdownRemark.edges

    posts.forEach(edge => {
      //Check if pages has a template key
      const locale = edge.node.frontmatter.locale
      if (edge.node.frontmatter.templateKey != null) {
        const id = edge.node.id
        createPage({
          path: edge.node.fields.slug,
          component: path.resolve(`src/templates/${String(edge.node.frontmatter.templateKey)}.tsx`),
          // additional data can be passed via context
          context: {
            id,
            locale,
          },
        })
      }
    })
  })
}

exports.onCreatePage = ({ page, actions }) => {
  const { createPage, deletePage } = actions

  return new Promise(resolve => {
    deletePage(page)

    Object.keys(locales).map(lang => {
      const localizedPath = locales[lang].default ? page.path : locales[lang].path + page.path

      return createPage({
        ...page,
        path: localizedPath,
        context: {
          locale: lang,
        },
      })
    })
    resolve()
  })
}

exports.onCreateNode = ({ node, actions, getNode }) => {
  const { createNodeField } = actions
  fmImagesToRelative(node) // convert image paths for gatsby images

  if (node.internal.type === `MarkdownRemark`) {
    const value = createFilePath({ node, getNode })
    createNodeField({
      name: `slug`,
      node,
      value,
    })
  }
}