Android - New Gboard dual language suggestions, how do I disable it?

How can I get back to manually switching between languages? Is there a setting I've missed, or do I have to roll back to an earlier version?

On the the keyboard options you'll find settings for Languages, Preferences, Theme... and in Text correction you'll find the toggle you're looking for: multilingual typing.

As you've mentioned, the multilingual suggestions is a new feature in Gboard (ref: Gboard, now available for Android - Official Google Blog.

Steps to enable/disable it:

  1. Open Gboard's Settings
  2. Select Languages
  3. Select a language
  4. On supported languages, below Language settings, tap Multilingual typing to enable/disable it. When enabled, you can check/uncheck other languages individually.

Note: the preference is not symmetric. When you have 2 languages that are supported for multilingual typings, you can disable it on language A while having it enabled on language B with A.

Quick access to Languages menu

In the latest versions of gboard the multilingual settings have moved to the Languages submenu which can be quickly accessed by long tapping the SPACE key, and then picking LANGUAGE SETTINGS in the popup that appears (at the very bottom, below the language list).

Now, tap each language one by one, and uncheck the multilingual typing option.

Access gboard Languages from the Settings

You can also access the Languages menu from settings:

Go to Settings > Languages & Input > Virtual keyboard > gboard > Languages