Does the prayer become invalid if a Muslim utters an unintentional word, such as saying "useless" instead of "Our Lord, to You be all praise," or changing a letter while reciting Al-Fatiha, such as the alif in "Ad-Dallin" turning into a ghayn?

Prayer is not invalidated merely by doubting one's speech or by substituting letters, for the default is that there should be no speech in prayer. However, if the worshiper is certain that he substituted the letter 'alif' with 'ghayn' in "al-ḍālīn" (the astray), pronouncing "al-ḍaghīn" (the malicious), or if he uttered "ghayru mujdin" (useless), then his prayer becomes invalid according to the jurists who hold that substituting a letter in Al-Fatihah changes the meaning, or that speech absolutely invalidates prayer. As for those who differentiate between the intentional and the ignorant or forgetful, his prayer is not invalidated.