How to set "Language Selector dropdown" required in node form
I found the solution here

You can enable i18n_node module that is included in i18n module. Then:

    Go to edit the content type (admin/structure/types/manage/page).
    Open the 'Multilingual settings' tab.
    Check the 'Require language' checkbox.

Done.


Multilingual content must be installed at first and then in content type, Multilingual settings tab will be added