SharePoint menus suddenly not changing language for users

SharePoint follows the browser language only if the Office 365 profile language has not been set. Several things can set this language. You can change it directly using Delve, or indirectly by setting a new language in your Microsoft 365 account settings or by changing it in AAD. To set it directly, go to your personal menu, click on My Microsoft 365 Profile
Click on Update Profile under your name
Scroll down to "How can I change language and regional settings?" and click on it
Click on the word "here"
click on the ellipsis (...) and then choose Language and Region
Under Language/Language Preference/My Display Languages, delete the current language
Click on Pick a new language and select the new language
Click on Add
Scroll to the bottom, and click "Save all and close"

If you are using out of the box SharePoint, then the setting will take a few minutes to a few hours to propagate to all SharePoint sites.