Add localized_compare

This is a helper to assist with sorting strings in a locale-aware
manner.

src/translations.cpp

@@ -686,3 +686,8 @@ std::string operator+( const translation &lhs, const translation &rhs )
 {
     return lhs.translated() + rhs.translated();
 }
+
+bool localized_comparator::operator()( const std::string &l, const std::string &r ) const
+{
+    return std::locale()( l, r );
+}

src/translations.h

@@ -244,4 +244,33 @@ std::string operator+( const translation &lhs, const std::string &rhs );
 std::string operator+( const std::string &lhs, const translation &rhs );
 std::string operator+( const translation &lhs, const translation &rhs );
 
+// Localized comparison operator, intended for sorting strings when they should
+// be sorted according to the user's locale.
+//
+// For convenience, it also sorts pairs recursively, because a common
+// requirement is to sort some list of objects by their names, and this can be
+// achieved by sorting a list of pairs where the first element of the pair is
+// the translated name.
+struct localized_comparator {
+    template<typename T, typename U>
+    bool operator()( const std::pair<T, U> &l, const std::pair<T, U> &r ) const {
+        if( ( *this )( l.first, r.first ) ) {
+            return true;
+        }
+        if( ( *this )( r.first, l.first ) ) {
+            return false;
+        }
+        return ( *this )( l.second, r.second );
+    }
+
+    template<typename T>
+    bool operator()( const T &l, const T &r ) const {
+        return l < r;
+    }
+
+    bool operator()( const std::string &, const std::string & ) const;
+};
+
+constexpr localized_comparator localized_compare;
+
 #endif // CATA_SRC_TRANSLATIONS_H