This tutorial is based on BlueStacks App Player for Windows v 0.9.0.4049 and higher
1) Click on the All Apps button(Shown below) and look for the BlueStacks Settings app.
2) Click / Tap on the BlueStacks Settings app to open it.
3) Now click the advanced setting as shown below
3) Click / Tap on the Languages & Input menu
4) Click / Tap on the Language option(shown below).
5) Scroll through the languages until you find the one you wish to use, e.g., Deutsch.
6) Click / Tap on the language you wish to select, e.g., Deutsch.
7) The selected language becomes the default language.