Hi, Bence Szabó.
Welcome to Microsoft support community.
We noticed you are having problem with customize Windows PE. By the way your English are pretty decent actually.
This kind of problem require a lot of special knowledge about the Windows PE while this community is designed for handle general Windows problem usually. So for handle your problem more professionally I would advise you to visit the: Windows - Microsoft Q&A and ask question there.
If you are ok with the limited help I could offer here are my personal suggestion:
Step 1: If you would like to add another language, you will need to install WinPE Optional Component. <-----
WinPE: Add packages (Optional Components Reference) | Microsoft Learn
<----- Deployment Image Servicing and Management (DISM) Best Practices | Microsoft Learn
Step 2: You should have a WinPE image (boot.wim).If not, see WinPE: Create bootable media to learn how to make a set of WinPE working files that include a WinPE image. You'll find boot.wim at sources\boot.wim in your working files.
Step 3: Added the language package with DISM /add-package ---- The package you have in your WinPE usually provide the language specific package you could install, for the system embedded element I do not have general idea yet.
Again, that is my brief understanding. Just for a general guideline. While above solution might not solve your problem instantly, I hope at least it will help you.
Have a good day.
Best Regards
Benjamin- MSFT | Microsoft Community Support Specialist