java.io.File error LanguageHandler class

package io.github.tivecs.lab.language;

import org.bukkit.Bukkit;
import org.bukkit.command.CommandSender;
import org.bukkit.entity.Player;

import java.io.File;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.UUID;

public class LanguageHandler {

    private HashMap<String, Language> registeredLanguages = new HashMap<>();

    private HashMap<UUID, String> userSelectedLanguage = new HashMap<>();

    private Language defaultLanguage = null;

    public LanguageHandler(){ }
    public LanguageHandler(File languageFolder){
        loadLanguages(languageFolder);
    }

    //-----------------------------------

    @SuppressWarnings("unchecked")
    public void sendMessage(UUID userUniqueId, String messagePath){
        Player onlineUser = Bukkit.getPlayer(userUniqueId);
        if (onlineUser.isOnline()) {
            if (userSelectedLanguage.get(userUniqueId) == null) {
                if (registeredLanguages.size() > 0) {
                    List<String> s = new ArrayList<>(registeredLanguages.keySet());
                    userSelectedLanguage.put(userUniqueId, s.get(0));
                } else {
                    return;
                }
            }

            Language lang = registeredLanguages.get(userSelectedLanguage.get(userUniqueId));
            List<String> msg = (List<String>) lang.getData().getOrDefault(messagePath, new ArrayList<>());

            for (String m : msg){
                onlineUser.sendMessage(m);
            }
        }
    }

    @SuppressWarnings("unchecked")
    public void sendMessage(CommandSender user, String messagePath){

    }

    //-----------------------------------
    public void registerUser(UUID userUniqueId, String langId){
        userSelectedLanguage.put(userUniqueId, langId);
    }

    public void loadLanguages(File parentDirectory){
        if (parentDirectory.isDirectory()){
            File[] langs = parentDirectory.listFiles();
            for (File f : langs){

                Language lang = new Language(f);
                lang.readAttribute();
                lang.readConfig();
                if (lang.isAsDefaultLanguage()){
                    defaultLanguage = lang;
                }
                registeredLanguages.put(lang.getLanguageId(), lang);
            }
        }
    }

}