flattener should try to render custom translations (#6540)

patches/server/0011-Adventure.patch

@@ -498,10 +498,10 @@ index 0000000000000000000000000000000000000000..eeedc30a45d9637d68f04f185b3dd90d
 +}
 diff --git a/src/main/java/io/papermc/paper/adventure/PaperAdventure.java b/src/main/java/io/papermc/paper/adventure/PaperAdventure.java
 new file mode 100644
-index 0000000000000000000000000000000000000000..f63b80a1120b2bf5f77f1c1edb928309a1272f79
+index 0000000000000000000000000000000000000000..41762b5ed19b17ee524f79726eaa4cf38479ba96
 --- /dev/null
 +++ b/src/main/java/io/papermc/paper/adventure/PaperAdventure.java
-@@ -0,0 +1,370 @@
+@@ -0,0 +1,375 @@
 +package io.papermc.paper.adventure;
 +
 +import com.mojang.brigadier.exceptions.CommandSyntaxException;
@@ -527,6 +527,7 @@ index 0000000000000000000000000000000000000000..f63b80a1120b2bf5f77f1c1edb928309
 +import net.kyori.adventure.translation.GlobalTranslator;
 +import net.kyori.adventure.util.Codec;
 +import net.minecraft.ChatFormatting;
++import net.minecraft.locale.Language;
 +import net.minecraft.nbt.CompoundTag;
 +import net.minecraft.nbt.ListTag;
 +import net.minecraft.nbt.StringTag;
@@ -545,7 +546,11 @@ index 0000000000000000000000000000000000000000..f63b80a1120b2bf5f77f1c1edb928309
 +    private static final Pattern LOCALIZATION_PATTERN = Pattern.compile("%(?:(\\d+)\\$)?s");
 +    public static final ComponentFlattener FLATTENER = ComponentFlattener.basic().toBuilder()
 +        .complexMapper(TranslatableComponent.class, (translatable, consumer) -> {
-+            final @NonNull String translated = net.minecraft.locale.Language.getInstance().getOrDefault(translatable.key());
++            if (!Language.getInstance().has(translatable.key())) {
++                consumer.accept(GlobalTranslator.render(translatable, Locale.US));
++                return;
++            }
++            final @NonNull String translated = Language.getInstance().getOrDefault(translatable.key());
 +
 +            final Matcher matcher = LOCALIZATION_PATTERN.matcher(translated);
 +            final List<Component> args = translatable.args();