feat: i18n user interface (#40306)

Co-authored-by: Oliver Eyton-Williams <ojeytonwilliams@gmail.com>

client/i18n/allLangs.js

@@ -0,0 +1,38 @@
+/* An error will be thrown if the CLIENT_LOCALE and CURRICULUM_LOCALE variables
+ * from the .env file aren't found in their respective arrays below
+ */
+const availableLangs = {
+  client: ['english', 'espanol'],
+  curriculum: ['english', 'chinese']
+};
+
+// Each client language needs an entry in the rest of the variables below
+
+/* These strings set the i18next langauge. It needs to be the two character
+ * string for the language to take advantage of available functionality.
+ * Use a 639-1 code here https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes
+ */
+const i18nextCodes = {
+  english: 'en',
+  espanol: 'es'
+};
+
+// These are for the language selector dropdown menu in the footer
+const langDisplayNames = {
+  english: 'English',
+  espanol: 'Español'
+};
+
+/* These are for formatting dates and numbers. Used with JS .toLocaleString().
+ * There's an example in profile/components/Camper.js
+ * List: https://github.com/unicode-cldr/cldr-dates-modern/tree/master/main
+ */
+const langCodes = {
+  english: 'en-US',
+  espanol: 'es-419'
+};
+
+exports.availableLangs = availableLangs;
+exports.i18nextCodes = i18nextCodes;
+exports.langDisplayNames = langDisplayNames;
+exports.langCodes = langCodes;