Hi @johnjohn-0427,
As a site owner or site collection administrator, you can use the Multiple Language User Interface (MUI) feature to offer individual users the ability to change the display language for their site user interface.
So the answer to your question "but i think the language should be changed from the sharepoint UI and not for each user.. am i correct?" is NO.
First issue:
Add your account to translators group firstly and then clear browser cache and cookies, reopen the browser and sign in the site. See this: https://support.microsoft.com/en-us/office/create-multilingual-sharepoint-sites-pages-and-news-2bb7d610-5453-41c6-a0e8-6f40b3ed750c#bkmk_muitranslations.
Second issue:
As @Domingos Massissa said, users have to change the display language for their personal use of the user interface. See this: https://support.microsoft.com/en-us/office/change-your-personal-language-and-region-settings-caa1fccc-bcdb-42f3-9e5b-45957647ffd7.
----------
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.