Add a language switcher with English, Italian, Spanish and French

The interface is translated through @nuxtjs/i18n: strings live in
i18n/locales/*.json, routes are prefixed except for the default English, and
useLocaleHead emits the lang attribute, the hreflang alternates and og:locale.

The switcher is a native select rather than a custom dropdown: keyboard
support, the platform picker on mobile and correct labelling come for free.

Article content stays single-language — this translates the site chrome only.

frontend/i18n/locales/en.json

@@ -0,0 +1,50 @@
+{
+  "site": {
+    "tagline": "Articles, guides and analysis.",
+    "skipToContent": "Skip to content",
+    "mainNavigation": "Main navigation",
+    "languageLabel": "Language",
+    "allArticles": "All articles",
+    "home": "Home",
+    "browse": "Browse",
+    "categories": "Categories",
+    "contact": "Contact",
+    "noCategories": "Nothing published yet",
+    "rights": "All rights reserved."
+  },
+  "home": {
+    "latest": "Latest",
+    "readArticle": "Read the article",
+    "moreStories": "More stories",
+    "viewAll": "View all articles"
+  },
+  "list": {
+    "archive": "The archive",
+    "articles": "Articles",
+    "articlesPage": "Articles — page {page}",
+    "archiveDescription": "Every article published on the blog.",
+    "category": "Category",
+    "categoryDescription": "Every article in the {name} category.",
+    "loading": "Loading…",
+    "empty": "Nothing has been published yet.",
+    "categoryEmpty": "There are no articles in this category.",
+    "failed": "Articles could not be loaded. Please try again later."
+  },
+  "pagination": {
+    "label": "Pagination",
+    "previous": "Previous page",
+    "next": "Next page",
+    "position": "Page {page} of {total}"
+  },
+  "article": {
+    "breadcrumb": "Breadcrumb",
+    "back": "Back to all articles"
+  },
+  "error": {
+    "notFoundTitle": "Page not found",
+    "notFoundBody": "The page you are looking for does not exist or has moved.",
+    "genericTitle": "Something went wrong",
+    "genericBody": "Please try again in a moment.",
+    "backHome": "Back to home"
+  }
+}