Use xx-XX format for translation files and default to en-US

Many languages have different variants, so instead of "en" we should be
using "en-US" - this will make it possible to add slightly different
British English translations, for example.
Note that this was already supported for the `locale` config option, we
were simply discarding the part after the dash when looking for the
right translation file.

Also make sure the en-US translations are actually American English,
I'll add en-GB separately.

src/config.js

@@ -17,7 +17,7 @@ const DEFAULT_CONFIG = {
     minAccuracy: null,
   },
   ignorePingLocation: true,
-  locale: "en",
+  locale: "en-US",
   map: {
     attribution:
       '&copy; <a href="https://osm.org/copyright">OpenStreetMap</a> contributors',

src/i18n.js

@@ -16,8 +16,8 @@ locales.keys().forEach((key) => {
 });
 
 export default new VueI18n({
-  locale: config.locale.split("-")[0],
-  fallbackLocale: "en",
+  locale: config.locale,
+  fallbackLocale: "en-US",
   formatFallbackMessages: true,
   messages,
 });

src/locales/en-US.json

@@ -10,7 +10,7 @@
   "Select user": "Select user",
   "Show all": "Show all",
   "Select device": "Select device",
-  "Distance travelled": "Distance travelled",
+  "Distance travelled": "Distance traveled",
   "Download raw data": "Download raw data",
   "Information": "Information",
   "Show last known locations": "Show last known locations",