I just installed Endeavour (KDE Plasma) and i installed everything without issues. But even though I set the language to be in Spanish in the installer, most of my apps are displayed in English (especially ones I installed, for example LibreOffice or Blender are in English).
I know that some distros offer a gui tool to set languages (like Ubuntu or Manjaro), is there a way to do that on Endeavour/Arch?
It’s not KDE bug. System settings on regional settings affect the LC_* env vars, which is the way Linux-based OS handles internationalization i18n. It’s up to a program whether or not they want to support internationalization i18n. DE surely supports i18n, but only some applications support i18n.