Hi there!
Thanks for your query.
Did you mean you want to translate/change the "Login" And "Sign Up" text? Then you can just change the text from the site module named "Menu Buttons". Here is a screenshot for your ease:
https://prnt.sc/c2fJeCCVKWvT
https://prnt.sc/243ra4Pi4cht
You can change login and registration title & image from the modules indicated here: https://prnt.sc/F6NyYcAJ3Yb7
If you want to translate the forms then please follow this general documentation and create a translation file for your default language.
Please go to this location: /language/en-GB/com_users.ini , open it with a text editor and change the desired texts inside.
Best Regards