This doesn't directly answers your question, but this explains how to use several keyboards/dictionaries at the same time and how to create custom keyboard/dictionaries.
Another Stackoverflow question about using multiple keyboards on Android: https://stackoverflow.com/questions/20197421/adding-a-second-dictionary-to-the-spell-check-in-android-4-3/44033793#44033793
Here is how for example to have a French keyboard with French dictionary + French keyboard with English dictionary
Android settings > Language & Input > Gboard (Google default keyboard).
Languages, you can tick all the keyboards you want. In my case,
French (France) and
This will give you the usual AZERTY French keyboard and the usual QWERTY English keyboard.
If you want the English dictionary with a French AZERTY keyboard, you can go to
Preferences > Personalised input styles and create a new keyboard
Language: English (UK) Layout: AZERTY, then in the
Languages menu, you will have a new keyboard in the list,
English (UK) (AZERTY). Tick it.
You now have 3 keyboards in your list:
English (UK) and
English (UK) (AZERTY).
While using the keyboard, you can then switch from one keyboard to another either with the special button or by holding the
Space bar. The
Space bar also shows the current language being used.
Using multiple dictionaries at the same time: In
Text correction, there is an option called "multilingual suggestions", this will use several dictionaries at the same time, recognising as valid all words from the selected languages. If active, the
Space bar will show something like FR-EN, to indicate that several languages are active at the same time. I personally don't like it.