This is the notation of the (pretty old) Plugin qtranslate. It was abandoned way back and replaced by mqtranslate and later qtranslate-x, which at this point are also abandoned. You can however convert your content to be used with wp multilang, which works with gutenberg, by replacing the <!--:lang--> tags with kinda shortcodes [:lang]. So for the english text you would use [:en]English Text[:].
It would be even better if you would put all languages within a post into the same language “shortcodes” instead of “opening” one language, closing it, and “opening” the next language. Like this:
[:en]English Text[:es]Spanish Text[:]