As far as I know,application internationalization needs to configure a file for each language separately, and configure all the content that needs to be displayed in the configuration file to the corresponding language.
Official tutorial on creating multilingual spfx:https://learn.microsoft.com/en-us/sharepoint/dev/spfx/web-parts/guidance/localize-web-parts
User profile service needs to wait for a low-priority timer job to synchronize the User Profile in each site collection, so it needs some time.
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