I'll try to help with this one:
First, obviously, enable the extension in Code's Preferences.
Now, right-clicking inside a document will let you choose from one of the pre-existing languages:
If, like me, you might need a language that is not in the list, you can easely add it if you have the available dictonary files. The extension 'looks' for available ...
Open terminal and type
$ sudo apt install ibus-avro
Type in terminal sudo im-config and select OK then Yes
Select Ibus as input method for the system
Click Ok and Ok for the next windows. Then restart your computer.
Type in terminal ibus-setup and remember your keyboard shortcut to switch layout, in our case it's Ctrl + Space
Choose Input Method tab and ...
Please note Keyobard layout and Input Method are seperated. Keyboard layout should follow your physical keyboard. Then add the required Input Method.
Also, for the Japanese keyboard layout found in eOS. I found no way to do transform and only Katakana can be inputted.
For adding Input Method, please follow these steps.
System Settings > Keyboard
Go to system settings, then "language & region" and use the "plus symbol" in the lower left of the window to add Japanese as an language. Set region and format as you like. These steps might be optional, I didn't try.
Then click to keyboard settings (lower most left big button). Again click the "plus symbol" on the lower left and search for japanese. ...
There are two ways you can change the language setting:
change Keyboard Which you can change by Alt+Shift
change input method Which you can change by Alt+Space(By default disabled)
Option one is easy even you can change at login-screen, But if you changed input-method this you can't change at login-screen using Alt+Space because ibus is not running by ...
System Settings > Keyboard > + in the lower-right corner > Select Spanish and click Add Layout.
Then you'll be able to switch between your current layout and Spanish layout in the menubar of via shortcut.
Visit the Languages @ elementary Weblate to get an overview of the languages supported in elementary OS. If you want to view the support on a per app basis, go to the Projects @ elementary Weblate. If you want to improve the translations, give this Translation Guide a read.