@Nuno has a verbose problem:
I have loaded my own database of Polish words containing over 4 million entries
This number is hard to believe.
An article in Wikipedia reports only 578,000 English words in the authoritative English dictionary. I have zero knowledge of the Polish language, so your count may include past and future tenses, different verb conjugations, plural and singular versions, and perhaps gender adjusted forms. I admit, I do not know. Yet, four million still seems quite high even accounting for variations.
See -> Dictionary Word Count
So I am not sure what problem you want to attack first?
Perhaps you can evaluate your word list and determine how long it takes to load the entire list? Are there better ways to optimize this list and load into your application?
You seemed to uncover a second problem, too. Your application loads the large word file, and allows the user to play a game.
If the user wishes to play another game, the large word file is reloaded. This is a structure problem.
If this is the problem you want to solve, you attack this as a logic problem. Or perhaps this is an architecture problem. Consider how you have structured your application.
In one respect, your application should load potential words into a giant, searchable array. This array is global and stays around for the entire time your application is running.
A sub section of your application is the game. At the start of each game, clear all your game's state. That is, clear the chosen word, clear the score, clear the used letters, clear the messages, clear the counters, clear everything! Clear JUST the game's state. Leave the global list of words untouched! Leave the sound effects untouched.
Then grab a word from the global word array, and copy it INTO your game structure. Start the game and allow your player to make guesses, updating states and messages along the way. When the game is over, then reset the game.
Do not reset your application! You want to keep the large word array, and only reset the game state.
Hope this helps.