Missing language now presents to the UI.

src/main/java/net/minecraftforge/fml/ForgeI18n.java

@@ -38,11 +38,20 @@ public class ForgeI18n {
 
     static {
         customFactories = new HashMap<>();
+        // {0,modinfo,id} -> modid from ModInfo object; {0,modinfo,name} -> displayname from ModInfo object
         customFactories.put("modinfo", (name, formatString, locale) -> new CustomReadOnlyFormat((stringBuffer, objectToParse) -> parseModInfo(formatString, stringBuffer, objectToParse)));
+        // {0,lower} -> lowercase supplied string
         customFactories.put("lower", (name, formatString, locale) -> new CustomReadOnlyFormat((stringBuffer, objectToParse) -> stringBuffer.append(StringUtils.toLowerCase((String)objectToParse))));
+        // {0,upper> -> uppercase supplied string
         customFactories.put("upper", (name, formatString, locale) -> new CustomReadOnlyFormat((stringBuffer, objectToParse) -> stringBuffer.append(StringUtils.toUpperCase((String)objectToParse))));
+        // {0,exc,class} -> class of exception; {0,exc,msg} -> message from exception
         customFactories.put("exc", (name, formatString, locale) -> new CustomReadOnlyFormat((stringBuffer, objectToParse) -> parseException(formatString, stringBuffer, objectToParse)));
+        // {0,vr} -> transform VersionRange into cleartext string using fml.messages.version.restriction.* strings
         customFactories.put("vr", (name, formatString, locale) -> new CustomReadOnlyFormat(((stringBuffer, o) -> MavenVersionStringHelper.parseVersionRange(formatString, stringBuffer, o))));
+        // {0,i18n,fml.message} -> pass object to i18n string 'fml.message'
+        customFactories.put("i18n", (name, formatString, locale) -> new CustomReadOnlyFormat((stringBuffer, o) -> stringBuffer.append(ForgeI18n.parseMessage(formatString, o))));
+        // {0,ornull,fml.absent} -> append String value of o, or i18n string 'fml.absent' (message format transforms nulls into the string literal "null")
+        customFactories.put("ornull", ((name, formatString, locale) -> new CustomReadOnlyFormat((stringBuffer, o) -> stringBuffer.append(Objects.equals(String.valueOf(o),"null") ? ForgeI18n.parseMessage(formatString) : String.valueOf(o)))));
     }
 
     private static void parseException(final String formatString, final StringBuffer stringBuffer, final Object objectToParse) {

src/main/java/net/minecraftforge/fml/loading/LanguageLoadingProvider.java

@@ -21,6 +21,7 @@ package net.minecraftforge.fml.loading;
 
 import net.minecraftforge.common.ForgeVersion;
 import net.minecraftforge.fml.language.IModLanguageProvider;
+import net.minecraftforge.fml.loading.moddiscovery.ExplodedDirectoryLocator;
 import net.minecraftforge.fml.loading.moddiscovery.ModFile;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
@@ -29,11 +30,7 @@ import org.apache.maven.artifact.versioning.DefaultArtifactVersion;
 import org.apache.maven.artifact.versioning.VersionRange;
 
 import java.nio.file.Path;
-import java.util.ArrayList;
+import java.util.*;
-import java.util.HashMap;
-import java.util.List;
-import java.util.Map;
-import java.util.ServiceLoader;
 import java.util.function.Consumer;
 import java.util.stream.Stream;
 
@@ -93,13 +90,16 @@ public class LanguageLoadingProvider
         serviceLoader.reload();
     }
 
-    public IModLanguageProvider findLanguage(String modLoader, VersionRange modLoaderVersion) {
+    public IModLanguageProvider findLanguage(ModFile mf, String modLoader, VersionRange modLoaderVersion) {
+        final String languageFileName = mf.getLocator() instanceof ExplodedDirectoryLocator ? "in-development" : mf.getFileName();
         final ModLanguageWrapper mlw = languageProviderMap.get(modLoader);
         if (mlw == null) {
-            throw new MissingLanguageException("Missing language "+modLoader);
+            LOGGER.error("Missing language {} version {} wanted by {}", modLoader, modLoaderVersion, languageFileName);
+            throw new EarlyLoadingException("Missing language "+modLoader, null, Collections.singletonList(new EarlyLoadingException.ExceptionData("fml.language.missingversion", modLoader, modLoaderVersion, languageFileName, "null")));
         }
         if (!modLoaderVersion.containsVersion(mlw.getVersion())) {
-            throw new MissingLanguageException("Missing language "+ modLoader + " matching range "+modLoaderVersion + " found "+mlw.getVersion());
+            LOGGER.error("Missing language {} version {} wanted by {}, found {}", modLoader, modLoaderVersion, languageFileName, mlw.getVersion());
+            throw new EarlyLoadingException("Missing language "+ modLoader + " matching range "+modLoaderVersion + " found "+mlw.getVersion(), null, Collections.singletonList(new EarlyLoadingException.ExceptionData("fml.language.missingversion", modLoader, modLoaderVersion, languageFileName, mlw.getVersion())));
         }
 
         return mlw.getModLanguageProvider();

src/main/java/net/minecraftforge/fml/loading/MissingLanguageException.java

@@ -1,28 +0,0 @@
-/*
- * Minecraft Forge
- * Copyright (c) 2016-2018.
- *
- * This library is free software; you can redistribute it and/or
- * modify it under the terms of the GNU Lesser General Public
- * License as published by the Free Software Foundation version 2.1
- * of the License.
- *
- * This library is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
- * Lesser General Public License for more details.
- *
- * You should have received a copy of the GNU Lesser General Public
- * License along with this library; if not, write to the Free Software
- * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
- */
-
-package net.minecraftforge.fml.loading;
-
-public class MissingLanguageException extends RuntimeException
-{
-    public MissingLanguageException(String message)
-    {
-        super(message);
-    }
-}

src/main/java/net/minecraftforge/fml/loading/ModSorter.java

@@ -55,6 +55,7 @@ public class ModSorter
         final ModSorter ms = new ModSorter(mods);
         EarlyLoadingException earlyLoadingException = null;
         try {
+            ms.findLanguages();
             ms.buildUniqueList();
             ms.verifyDependencyVersions();
             ms.sort();
@@ -65,6 +66,10 @@ public class ModSorter
         return LoadingModList.of(ms.modFiles, ms.sortedList, earlyLoadingException);
     }
 
+    private void findLanguages() {
+        modFiles.stream().forEach(mf->mf.identifyLanguage());
+    }
+
     private void sort()
     {
         final TopologicalSort.DirectedGraph<Supplier<ModFileInfo>> topoGraph = new TopologicalSort.DirectedGraph<>();
@@ -136,8 +141,7 @@ public class ModSorter
         if (!missingVersions.isEmpty()) {
             final List<EarlyLoadingException.ExceptionData> exceptionData = missingVersions.stream().map(mv ->
                     new EarlyLoadingException.ExceptionData("fml.modloading.missingdependency", mv.getModId(),
-                            mv.getOwner().getModId(), mv.getVersionRange(),
+                            mv.getOwner().getModId(), mv.getVersionRange(), modVersions.getOrDefault(mv.getModId(), new DefaultArtifactVersion("null")))).
-                            modVersions.containsKey(mv.getModId()) ? modVersions.get(mv.getModId()) : "NONE" )).
                     collect(Collectors.toList());
             throw new EarlyLoadingException("Missing mods", null, exceptionData);
         }

src/main/java/net/minecraftforge/fml/loading/moddiscovery/ModFile.java

@@ -73,6 +73,10 @@ public class ModFile
         return locator.findPath(this, className);
     }
 
+    public void identifyLanguage() {
+        this.loader = FMLLoader.getLanguageLoadingProvider().findLanguage(this, this.modFileInfo.getModLoader(), this.modFileInfo.getModLoaderVersion());
+    }
+
     public enum Type {
         MOD, LIBRARY, LANGPROVIDER
     }
@@ -124,7 +128,6 @@ public class ModFile
         this.coreMods = ModFileParser.getCoreMods(this);
         this.coreMods.forEach(mi-> LOGGER.debug(LOADING,"Found coremod {}", mi.getPath()));
         this.accessTransformer = locator.findPath(this, "META-INF", "accesstransformer.cfg");
-        this.loader = FMLLoader.getLanguageLoadingProvider().findLanguage(this.modFileInfo.getModLoader(), this.modFileInfo.getModLoaderVersion());
         return true;
     }
 

src/main/resources/assets/forge/lang/en_us.json

@@ -23,8 +23,11 @@
   "fml.modloading.failedtoloadmod":"{0,modinfo,name} ({0,modinfo,id}) has failed to load correctly\n\u00a77{2,exc,msg}",
   "fml.modloading.errorduringevent":"{0,modinfo,name} ({0,modinfo,id}) encountered an error during the {1,lower} event phase\n\u00a77{2,exc,msg}",
   "fml.modloading.failedtoloadforge": "Failed to load forge",
-  "fml.modloading.missingdependency": "Mod {4} has missing dependency {3}\n\u00a77Want {5,vr}, have {6}",
+  "fml.modloading.missingdependency": "Mod {4} has missing dependency {3}\n\u00a77Want {5,vr}, have {6,i18n,fml.messages.artifactversion}",
+  "fml.language.missingversion": "Mod File {5} needs language provider {3}:{4,vr}\n\u00a77Found {6,i18n,fml.messages.artifactversion}",
 
+  "fml.messages.artifactversion":"{0,ornull,fml.messages.artifactversion.none}",
+  "fml.messages.artifactversion.none":"none",
   "fml.messages.version.restriction.any":"any",
   "fml.messages.version.restriction.lower.inclusive":"{0} or above",
   "fml.messages.version.restriction.lower.exclusive":"above {0}",