feat(gatsby-source-contentful): add locale filter (#12939)

* Added locale filter when creating nodes

* Updated test locale and snapshot

* Updated from PR comments, moved filtering function earlier in the flow

* New line between function

* add note where to get locale codes

packages/gatsby-source-contentful/README.md

@@ -144,6 +144,14 @@ Downloads and caches `ContentfulAsset`'s to the local filesystem. Allows you to
 
 You can pass in any other options available in the [contentful.js SDK](https://github.com/contentful/contentful.js#configuration).
 
+**`localeFilter`** [function][optional] [default: `() => true`]
+
+Possibility to limit how many locales/nodes are created in graphQL. This can limit the memory usage by reducing the amout of nodes created. Useful if you have a large space in contentful and only want to get the data from one selected locale.
+
+For example, to filter locales on only germany `localeFilter: locale => locale.code === 'de-DE'`
+
+List of locales and their codes can be found in Contentful app -> Settings -> Locales
+
 ## Notes on Contentful Content Models
 
 There are currently some things to keep in mind when building your content models at Contentful.

packages/gatsby-source-contentful/src/fetch.js

@@ -23,6 +23,7 @@ module.exports = async ({ spaceId, syncToken, ...options }) => {
     console.log(`Fetching default locale`)
     locales = await client.getLocales().then(response => response.items)
     defaultLocale = _.find(locales, { default: true }).code
+    locales = locales.filter(options.localeFilter || (() => true))
     console.log(`default locale is : ${defaultLocale}`)
   } catch (e) {
     console.log(