You're free to enter any input sentence you want in the text box.

The input sentence generally should be in the language you selected from the dropdown. For example, if the dropdown has "French" selected you could enter the text "Allons enfants de la Patrie, Le jour de gloire est arrivé!"

Clicking "Submit" then generates a TTS reading of the sentence you input in the language selected from the dropdown.

For fun you can mix and match. In other words, select a language from the drop down and enter text in the text box not in the language selected from the dropdown. (For example, the dropdown could have "French" selected and the sentence could be "O say can you see, by the dawn's early light". This gives interesting results, it sounds as if a native French speaker is speaking English.)