External interface language switcher

### Describe the problem

Not signed in user cannot change the interface of the current instance.
And I prefer an external language switcher. That is more convenient to change the interface language.

### Describe the solution you'd like

Add an external interface language switcher, like the one on weblate.org.
And the interface language setting of the singed-in user should be changed accordingly.

### Describe alternatives you've considered

_No response_

### Screenshots

The interface language switcher on weblate.org

### Additional context

_No response_