Since today, the Debian packaging has changed in weechat.org Debian/Ubuntu/Raspbian repositories: the scripting plugins are split into multiple packages, so that Perl, Python, Ruby, … can be installed separately.
Important: if you are using the WeeChat development packages from weechat.org, you will have to install manually the scripting packages.
For more information: https://weechat.org/blog/post/2017/11/04/Split-of-scripting-Debian-packages
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