Now this website is powered by WP 4.9.5 and xili-language 2.22.10.

Where is xili-language 2.22.10 downloadable ?

in xiligroup GitHub.

What is the next target ?

Cleaning the code (some parts are 10 years old !) to ship 3.0 !!

Is xili-language compatible with Gutenberg ?

Yes since Gutenberg version 2.7 (April 18th 2018). Gutenberg now follow the rules to display (or not) custom taxonomies as language used by xili-language since 2009 in admin/editor post edit screen.