If this happens it's usually solved by simply logging out of Spotify and logging in again. If it still doesn't work, it's a service disturbance and Spotify will make sure to address the issue as quickly as possible.
How do I log in if my user name contains non-Latin characters?
Navigate to Settings > General > Languages
Add "Additional Keyboard" with the language you need
Open the Spotify app
Click the "Enter" key to open the virtual keyboard
Change the language with the "Globes" icon and enter the user name