The bug

Change your language and then try to create a new superflat world, when looking under presets you will see that the preset names are still displayed in the previously selected language. Only after a restart of Minecraft those strings display the correct translations.

How to reproduce

1. Switch to any non en_us language
2. Go to the superflat world screen, and then open the presets screen
   → Observe that the preset names are in that language
3. Switch to en_us language
4. Go to the superflat world screen, and then open the presets screen
   → Observe that the preset names are still in that language
5. Restart Minecraft
6. Go to the superflat world screen, and then open the presets screen
   → Observe that the preset names are in English now

This issue also affected the customized screen, which was removed in 1.13.

Cause

The preset names are added in a static block within GuiFlatPresets (and formerly GuiScreenCustomizePresets), but they are added with the name already translated (I18n.format is called within the static block). The name should be translated when the text is rendered, instead.