Use rootian locale for case conversion.

Thank you to Spottedleaf.

src/main/java/com/mojang/brigadier/CommandDispatcher.java

@@ -20,6 +20,7 @@ import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.LinkedHashSet;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.CompletableFuture;
@@ -588,12 +589,13 @@ public class CommandDispatcher<S> {
 
         final String fullInput = parse.getReader().getString();
         final String truncatedInput = fullInput.substring(0, cursor);
+        final String truncatedInputLowerCase = truncatedInput.toLowerCase(Locale.ROOT);
         @SuppressWarnings("unchecked") final CompletableFuture<Suggestions>[] futures = new CompletableFuture[parent.getChildren().size()];
         int i = 0;
         for (final CommandNode<S> node : parent.getChildren()) {
             CompletableFuture<Suggestions> future = Suggestions.empty();
             try {
-                future = node.listSuggestions(context.build(truncatedInput), new SuggestionsBuilder(truncatedInput, start));
+                future = node.listSuggestions(context.build(truncatedInput), new SuggestionsBuilder(truncatedInput, truncatedInputLowerCase, start));
             } catch (final CommandSyntaxException ignored) {
             }
             futures[i++] = future;

src/main/java/com/mojang/brigadier/arguments/BoolArgumentType.java

@@ -34,10 +34,10 @@ public class BoolArgumentType implements ArgumentType<Boolean> {
 
     @Override
     public <S> CompletableFuture<Suggestions> listSuggestions(final CommandContext<S> context, final SuggestionsBuilder builder) {
-        if ("true".startsWith(builder.getRemaining().toLowerCase())) {
+        if ("true".startsWith(builder.getRemainingLowerCase())) {
             builder.suggest("true");
         }
-        if ("false".startsWith(builder.getRemaining().toLowerCase())) {
+        if ("false".startsWith(builder.getRemainingLowerCase())) {
             builder.suggest("false");
         }
         return builder.buildFuture();

src/main/java/com/mojang/brigadier/suggestion/SuggestionsBuilder.java

@@ -8,18 +8,27 @@ import com.mojang.brigadier.context.StringRange;
 
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Locale;
 import java.util.concurrent.CompletableFuture;
 
 public class SuggestionsBuilder {
     private final String input;
+    private final String inputLowerCase;
     private final int start;
     private final String remaining;
+    private final String remainingLowerCase;
     private final List<Suggestion> result = new ArrayList<>();
 
-    public SuggestionsBuilder(final String input, final int start) {
+    public SuggestionsBuilder(final String input, final String inputLowerCase, final int start) {
         this.input = input;
+        this.inputLowerCase = inputLowerCase;
         this.start = start;
         this.remaining = input.substring(start);
+        this.remainingLowerCase = inputLowerCase.substring(start);
+    }
+
+    public SuggestionsBuilder(final String input, final int start) {
+        this(input, input.toLowerCase(Locale.ROOT), start);
     }
 
     public String getInput() {
@@ -34,6 +43,10 @@ public class SuggestionsBuilder {
         return remaining;
     }
 
+    public String getRemainingLowerCase() {
+        return remainingLowerCase;
+    }
+
     public Suggestions build() {
         return Suggestions.create(input, result);
     }
@@ -74,10 +87,10 @@ public class SuggestionsBuilder {
     }
 
     public SuggestionsBuilder createOffset(final int start) {
-        return new SuggestionsBuilder(input, start);
+        return new SuggestionsBuilder(input, inputLowerCase, start);
     }
 
     public SuggestionsBuilder restart() {
-        return new SuggestionsBuilder(input, start);
+        return createOffset(start);
     }
 }

src/main/java/com/mojang/brigadier/tree/LiteralCommandNode.java

@@ -16,15 +16,18 @@ import com.mojang.brigadier.suggestion.SuggestionsBuilder;
 
 import java.util.Collection;
 import java.util.Collections;
+import java.util.Locale;
 import java.util.concurrent.CompletableFuture;
 import java.util.function.Predicate;
 
 public class LiteralCommandNode<S> extends CommandNode<S> {
     private final String literal;
+    private final String literalLowerCase;
 
     public LiteralCommandNode(final String literal, final Command<S> command, final Predicate<S> requirement, final CommandNode<S> redirect, final RedirectModifier<S> modifier, final boolean forks) {
         super(command, requirement, redirect, modifier, forks);
         this.literal = literal;
+        this.literalLowerCase = literal.toLowerCase(Locale.ROOT);
     }
 
     public String getLiteral() {
@@ -66,7 +69,7 @@ public class LiteralCommandNode<S> extends CommandNode<S> {
 
     @Override
     public CompletableFuture<Suggestions> listSuggestions(final CommandContext<S> context, final SuggestionsBuilder builder) {
-        if (literal.toLowerCase().startsWith(builder.getRemaining().toLowerCase())) {
+        if (literalLowerCase.startsWith(builder.getRemainingLowerCase())) {
             return builder.suggest(literal).buildFuture();
         } else {
             return Suggestions.empty();