Is there any way to change localization via query params?
Answer: No yet
And how to show the language select box in login page? (oxauth / identifier)
Answer: There is no drop down to select language for oxauth component. The multi-language can be supported by customizing the public facing login pages to meet your requirements. We will definitely consider implementing this feature if we hear more demand from customers.
For customization: https://gluu.org/docs/ce/3.1.2/operation/custom-design/
If you want to change the default language this ticket reply can help you.
In case you are interested in becoming a supported customer to request this feature, feel free to schedule a call with us.