A bug : Language drop-down switches back

[o-o-o]

I am experiencing a bug with Language drop down. Please advice.

I've installed a 2nd language, and it worked just fine. I switched language back and force using the Language drop down in standard default Cure theme and all worked as expected.

Then I've installed another theme which doesn't show the Language combo-box. So I went to the Account Settings and set different language as a preferred one (for Administrator). That helped with he previous issue and language switched to one that I set as preferred.

But now, if I switch back to default Curve theme, then Language drop down does not work as expected any more. I am selecting another language, but it switching back to preferred one    And the only way to change current UI language is to update the preferred language, which requires visiting of Account Settings, using drop down there, entering my password to save. A much longer procedure than before.

Any idea how to return it back to original behavior, when just a selection in drop-down of default SMF theme switches the UI language?