1. Start the online installer on a machine with German UI.
2. Enter Qt account data and click "Weiter" ("Next").
   I used one with a commercial license and this will affect the symptom. I assume for an opensource license, the symptom will be different but equally wrong.
   The installer greets you with the sentence: "Willkommen zum kommerziell Qt Setup." This is not a correct German sentence.

This is the result of a bad combination of translated strings:

tr("Welcome to the %1 Setup Wizard.").arg(productName())

This works in English because there the product name "commercial" coincidentally is the right form of an adjective. In other languages, e.g. German, it is not.