Description from extension meta
Compose Key for Chrome OS using Japanese keyboard
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Description from store
This is just a fork of the original Compose Key extension from Google, which enables Compose Key on a japanese layout, and only this one. To use the Compose Key with other layouts, please install the original extension instead.
To have it as a keyboard candidate, you need to have Japanese enabled as a language in your ChromeOS settings.
Instructions for installing and configuring the extension including a complete list of available key sequences can be found on the original extension repository : https://github.com/google/extra-keyboards-for-chrome-os/tree/master/composekey.
The current extension was neither developped by Google, nor endorsed by them in any way. It is only the result of a fork from the original code.