Throw if constructing a non-English multilingual message

This matches the behaviour for when converting from YAML, ensuring that
a non-English multilingual message can never be created.

src/backend/metadata/message.cpp

@@ -24,6 +24,7 @@
 
 #include "message.h"
 #include "../helpers/language.h"
+#include "../error.h"
 
 #include <boost/log/trivial.hpp>
 
@@ -42,7 +43,17 @@ namespace loot {
     }
 
     Message::Message(const unsigned int type, const std::vector<MessageContent>& content,
-                     const std::string& condition) : _type(type), _content(content), ConditionalMetadata(condition) {}
+                     const std::string& condition) : _type(type), _content(content), ConditionalMetadata(condition) {
+        if (content.size() > 1) {
+            bool englishStringExists = false;
+            for (const auto &mc : content) {
+                if (mc.Language() == loot::Language::english)
+                    englishStringExists = true;
+            }
+            if (!englishStringExists)
+                throw loot::error(error::invalid_args, "bad conversion: multilingual messages must contain an English content string");
+        }
+    }
 
     bool Message::operator < (const Message& rhs) const {
         if (!_content.empty() && !rhs.Content().empty())

src/tests/backend/metadata/message_test.h

@@ -71,6 +71,14 @@ namespace loot {
             EXPECT_EQ("condition1", message.Condition());
         }
 
+        TEST_P(MessageTest, vectorContentConstructorShouldThrowIfMultipleContentStringsAreGivenAndNoneAreEnglish) {
+            MessageContents contents({
+                MessageContent("content1", Language::german),
+                MessageContent("content2", Language::french),
+            });
+            EXPECT_ANY_THROW(Message(Message::error, contents, "condition1"));
+        }
+
         TEST_P(MessageTest, messagesWithDifferentContentStringsShouldBeUnequal) {
             Message message1(Message::say, "content1", "condition1");
             Message message2(Message::say, "content2", "condition1");