Hi @yskstdev,
Welcome to the Auth0 Community!
I understand you are looking to change the language on your login page.
As you have found, you will need to select your preferred language in your Tenant Settings and pass the ui_locales query parameter in your Authorize Request.
It should look something like the following:
https://YOUR_DOMAIN/authorize?
response_type=code&
client_id=YOUR_CLIENT_ID&
redirect_uri=https://YOUR_APP/callback&
scope=SCOPE&
audience=API_AUDIENCE&
state=STATE&
ui_locales=ja
Alternatively, if you know you will use only one language, you could deselect all languages in your Tenant Settings except for your preferred language. This way, it defaults to that language, and you will not need to pass the ui_locales query parameter.
Please let me know how this works for you or if you have any questions.
Thank you.