Added ability for language files to opt-in to loading using normal Java properties format.

This allows for escape characters, as well as keeping %d and %f replacements in tact.
To opt-in, the lang file must have this comment line:
# PARSE_ESCAPES
As this is opt-in, any errors resulting from misused format strings is on the modder/end user.

fml/patches/minecraft/net/minecraft/client/resources/Locale.java.patch

@@ -0,0 +1,11 @@
+--- ../src-base/minecraft/net/minecraft/client/resources/Locale.java
++++ ../src-work/minecraft/net/minecraft/client/resources/Locale.java
+@@ -107,6 +107,8 @@
+ 
+     private void func_135021_a(InputStream p_135021_1_) throws IOException
+     {
++        p_135021_1_ = net.minecraftforge.fml.common.FMLCommonHandler.instance().loadLanguage(field_135032_a, p_135021_1_);
++        if (p_135021_1_ == null) return;
+         Iterator iterator = IOUtils.readLines(p_135021_1_, Charsets.UTF_8).iterator();
+ 
+         while (iterator.hasNext())

fml/patches/minecraft/net/minecraft/util/StringTranslate.java.patch

@@ -1,6 +1,6 @@
 --- ../src-base/minecraft/net/minecraft/util/StringTranslate.java
 +++ ../src-work/minecraft/net/minecraft/util/StringTranslate.java
-@@ -18,16 +18,35 @@
+@@ -18,16 +18,38 @@
  {
      private static final Pattern field_111053_a = Pattern.compile("%(\\d+\\$)?[\\d\\.]*[df]");
      private static final Splitter field_135065_b = Splitter.on('=').limit(2);
@@ -35,10 +35,13 @@
          try
          {
 -            InputStream inputstream = StringTranslate.class.getResourceAsStream("/assets/minecraft/lang/en_US.lang");
++            inputstream = net.minecraftforge.fml.common.FMLCommonHandler.instance().loadLanguage(table, inputstream);
++            if (inputstream == null) return table;
++
              Iterator iterator = IOUtils.readLines(inputstream, Charsets.UTF_8).iterator();
  
              while (iterator.hasNext())
-@@ -42,17 +61,17 @@
+@@ -42,17 +64,17 @@
                      {
                          String s1 = astring[0];
                          String s2 = field_111053_a.matcher(astring[1]).replaceAll("%$1s");

fml/src/main/java/net/minecraftforge/fml/common/FMLCommonHandler.java

@@ -12,10 +12,18 @@
 
 package net.minecraftforge.fml.common;
 
+import java.io.ByteArrayInputStream;
 import java.io.File;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
 import java.lang.ref.WeakReference;
+import java.util.HashMap;
+import java.util.Iterator;
 import java.util.List;
 import java.util.Map;
+import java.util.Map.Entry;
+import java.util.Properties;
 import java.util.Set;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.ExecutionException;
@@ -50,6 +58,8 @@ import net.minecraftforge.fml.common.network.NetworkRegistry;
 import net.minecraftforge.fml.relauncher.Side;
 import net.minecraftforge.fml.server.FMLServerHandler;
 
+import org.apache.commons.io.Charsets;
+import org.apache.commons.io.IOUtils;
 import org.apache.logging.log4j.Level;
 import org.apache.logging.log4j.Logger;
 
@@ -685,4 +695,45 @@ public class FMLCommonHandler
             FMLLog.log(Level.FATAL, e, "Exception caught executing FutureTask: " + e.toString());
         }
     }
+
+    /**
+     * Loads a lang file, first searching for a marker to enable the 'extended' format {escape charaters}
+     * If the marker is not found it simply returns and let the vanilla code load things.
+     * The Marker is 'PARSE_ESCAPES' by itself on a line starting with '#' as such:
+     * #PARSE_ESCAPES
+     *
+     * @param table The Map to load each key/value pair into.
+     * @param inputstream Input stream containing the lang file.
+     * @return A new InputStream that vanilla uses to load normal Lang files, Null if this is a 'enhanced' file and loading is done.
+     */
+    public InputStream loadLanguage(Map<String, String> table, InputStream inputstream) throws IOException
+    {
+        byte[] data = IOUtils.toByteArray(new InputStreamReader(inputstream, Charsets.UTF_8));
+
+        boolean isEnhanced = false;
+        for (String line : IOUtils.readLines(new ByteArrayInputStream(data), Charsets.UTF_8))
+        {
+            if (line.charAt(0) == '#')
+            {
+                line = line.substring(1).trim();
+                if (line.equals("PARSE_ESCAPES"))
+                {
+                    isEnhanced = true;
+                    break;
+                }
+            }
+        }
+
+        if (!isEnhanced)
+            return new ByteArrayInputStream(data);
+
+        Properties props = new Properties();
+        props.load(new InputStreamReader(new ByteArrayInputStream(data), Charsets.UTF_8));
+        for (Entry e : props.entrySet())
+        {
+            table.put((String)e.getKey(), (String)e.getValue());
+        }
+        props.clear();
+        return null;
+    }
 }