When I open the launcher, there are splashes like "Wash your hands" that match the corona time. Where can I find these texts stored?
If you press the Windows key and "R" on your keyboard at the same time, a small window opens.
There you type in "% appdata%" and confirm.
(On Mac you have to go to… Use, hold Alt and go to library, then to Application support where the Minecraft folder is)
From there you open the "minecraft" folder, then the "versions" folder and then you select the version you are currently playing. There you will find a ZIP archive of the same name that you can only open with e.g. WinRAR.
From there it continues via "assets" → "minecraft" → "texts"
There you will find a text file called "splashes", which you can open and edit with Windows easy.
If you use a texture pack you have to make sure that this text file is hidden in the texture pack. You just have to open the folder of the texture pack and then you will find an "assets" folder there.
Finally, I want to emphasize that I strongly suspect that on special occasions the splash texts in the menu do not come from this text file, but are loaded online to respond to certain occasions, such as April 1, Christmas or even CoViD-19.
I agree. That's my problem.