What I expected to happen was:

The Superflat select preset language is in the language you currently have on

What actually happened was:

The Superflat select preset language is locked in the first language you changed Minecraft to, but the heading is in the current language

How to reproduce:

Open Minecraft

Change language

Go to the Superflat presets menu

Cancel out (return to the main menu, do NOT make a world)

Change language

Return to the Superflat presets menu

Change language

Return to the Superflat presets menu again