Where to report a bug in a keyboard app? I have Nexus4 with Android.
One of the input languages is Russian. I use Russian word suggestions or swype method for messaging.
There is a particular form of Russian verbs (frequently used) that seems to be omitted in the dictionary (suggestions).
Where can I report this bug?
If you're using the stock Android keyboard, you can report it on the central Android bug tracker. Be warned that there are already more than 25000 reports there, and many never receive any response from Google.

If you find a bug in a third-party keyboard you're using, visit the keyboard's page in Google Play (either through the Play Store app or on the website) and click Email developer.

Lastly, and this definitely isn't applicable in this case, if you find a bug in a device manufacturer's keyboard (such as the Samsung Keyboard found on Samsung phones), the answer will depend on which manufacturer, so you should contact their product support in the first instance. Hardware manufacturers tend not to have publicly-accessible bug trackers.