Turns out YPP does need getLocalePrefix(). :)

core/src/main/java/com/threerings/resource/ResourceManager.java

@@ -257,14 +257,22 @@ public class ResourceManager
      */
     public void setLocalePrefix (final String prefix)
     {
+        _localePrefix = prefix;
         setLocaleHandler(
             new LocaleHandler() {
+                public String getPrefix() {
+                    return prefix;
+                }
                 public String getLocalePath (String path) {
                     return PathUtil.appendPath(prefix, path);
                 }
             });
     }
 
+    public String getLocalePrefix () {
+        return _localePrefix;
+    }
+
     /**
      * Configure a custom LocaleHandler.
      */
@@ -1097,6 +1105,9 @@ public class ResourceManager
     /** Converts a path to a locale-specific path. */
     protected LocaleHandler _localeHandler;
 
+    /** Set when we have a simple prefix-based locale handler (needed for YPP). */
+    protected String _localePrefix;
+
     /** Maps resource paths to observed file resources. */
     protected HashMap<String, ObservedResource> _observed = Maps.newHashMap();