MySimpleStranslator

import java.util.HashMap;

public class Translator {
    private HashMap<String, String> dictionary = new HashMap<String, String>();

    public void addNewWord(String en, String ru) {
        dictionary.put(en.toLowerCase(), ru.toLowerCase());
    }

    public String translate(String en) {
        String[] arr = en.split("\\s");

        StringBuilder stringBuilder = new StringBuilder();
        StringBuilder sb_unknown_words = new StringBuilder();

        for (String anArr : arr) {
            if (dictionary.get(anArr) == null)
                sb_unknown_words.append(anArr).append("\n");
            else
                stringBuilder.append(dictionary.get(anArr) != null ? dictionary.get(anArr).concat(" ") : "");
        }

        if (sb_unknown_words.length() == 0)
            return stringBuilder.insert(0, "Русский:\n").toString();

        return stringBuilder.toString() + "\n" + new StringBuilder("В словаре нет подходящего перевода для:\n").append(sb_unknown_words.toString());
    }

}