add translation keys (#246)

* Fix i18n tsc issue

"t" can return null by default.
This was already disabled in the config, but it had no effect on tsc.

* Fix translation in "DefaultNavBar"

key was missing and since this is a constant the translation wouldn't update in case the language got changed after first load.

* Enable typing for i18n translation keys

* Add translation keys

src/i18n/i18next.d.ts

@@ -0,0 +1,9 @@
+import 'i18next';
+import resources from 'i18n/translations';
+
+declare module 'i18next' {
+    interface CustomTypeOptions {
+        returnNull: false;
+        resources: typeof resources.en;
+    }
+}