Added the ability for a user to define and set a custom locale

New environment variable CUSTOM_LOCALE allows a user to define a locale per the /public/locales directory (this should be listed in the docs, will create a pull request for that too).

If the environment variable is blank or invalid it reverts to previous behaviour of system + default locale. Fully tested the above as follows:

CUSTOM_LOCALE = 'nl' < This works correctly, translating to nl.
CUSTOM_LOCALE = 'HelloThere' < This reverts to previous behavior
CUSTOM_LOCALE = '' < Also reverts
#CUSTOM_LOCALE = < Also reverts

server/config.js

@@ -273,6 +273,11 @@ const conf = convict({
     default: '#003eaa',
     env: 'UI_COLOR_ACCENT'
   },
+  custom_locale: {
+    format: String,
+    default: '',
+    env: 'CUSTOM_LOCALE'
+  },
   ui_custom_assets: {
     android_chrome_192px: {
       format: String,

server/state.js

@@ -3,9 +3,18 @@ const layout = require('./layout');
 const assets = require('../common/assets');
 const getTranslator = require('./locale');
 const { getFxaConfig } = require('./fxa');
+const fs = require('fs');
+const path = require('path');
 
 module.exports = async function(req) {
-  const locale = req.language || 'en-US';
+  const locale = (() => {
+    if (config.custom_locale != '' && fs.existsSync(path.join(__dirname,'../public/locales',config.custom_locale))) {
+        return config.custom_locale;
+    }
+    else {
+      return req.language || 'en-US';
+    }
+  })();
   let authConfig = null;
   let robots = 'none';
   if (req.route && req.route.path === '/') {