How to enable multilingual typing in Google Keyboard (GBoard) app? Until recently I had multilingual typing enabled on my keyboard. It was possible to swipe in German and English at the same keyboard without changing languages. The spacebar had "English - German" written on it.

Somehow it got accidentally turned off. Maybe by some gesture on the spacebar?

Now I can't figure out how to turn that feature on again.
The toggle used to turn it on in the keyboard settings is disabled and can't be turned on.



The hint says that multiple languages have to be enabled. But where can that be done?

On the languages screen there are already two languages. Unfortunately they are also separate keyboards and I always have to switch with the globe button.
Likely reason is that the second language you want is not installed in language preferences


To do that go to System &rarr; Language and input &rarr;
language  &rarr;
preference  &rarr; add  language (the one you want to be displayed on keyboard).  If it is already there, remove and add ( You can tap and hold a language to change the order or remove language from the 3 dots menu on top right)
Now check as in first screenshot (of your question ) to see if multi lingual option is shown ( it should be). Enable the option and tick the language. 
Long press or slide on space key to reveal option as shown and select the bilingual switch ( it is Telugu in first snapshot and TE in second ). Your keyboard should show as desired both languages. If it doesn't reboot and check 





(Tap to enlarge)

Edit This didn't work for OP. Maybe the Gboard settings has Show Language switch key disabled (Language &amp; input &rarr;Virtual keyboard &rarr;Gboard &rarr; Preferences &rarr;Show language switch key) . Enable that. Also see in this connection https://android.stackexchange.com/a/165509

Edit2 OP found the setting to be enabled but problem continues so -  1. Update Gboard if available 2. If not, go to settings and from app info of Gboard clear cache and see if that helps; next, clear data of app ;also disable and enable app