Use rootian locale for case conversion.

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;
@@ -588,7 +589,7 @@ public class CommandDispatcher<S> {
 
         final String fullInput = parse.getReader().getString();
         final String truncatedInput = fullInput.substring(0, cursor);
-        final String truncatedInputLowerCase = truncatedInput.toLowerCase();
+        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()) {

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

@@ -8,6 +8,7 @@ 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 {
@@ -27,7 +28,7 @@ public class SuggestionsBuilder {
     }
 
     public SuggestionsBuilder(final String input, final int start) {
-        this(input, input.toLowerCase(), start);
+        this(input, input.toLowerCase(Locale.ROOT), start);
     }
 
     public String getInput() {

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

@@ -16,6 +16,7 @@ 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;
 
@@ -26,7 +27,7 @@ public class LiteralCommandNode<S> extends CommandNode<S> {
     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();
+        this.literalLowerCase = literal.toLowerCase(Locale.ROOT);
     }
 
     public String getLiteral() {