We use linuxdeploy and its Gtk plugin to build an AppImage on Ubuntu 20.04. Our package uses libgtksourceview-4-0 (which depends on libgtksourceview-4-common). While the gtksourceview library is installed automatically in the AppImage, the styles and language definitions are not:
~/Downloads/squashfs-root$ find ./usr -name *sourceview*
./usr/share/doc/libgtksourceview-4-0
./usr/lib/libgtksourceview-4.so.0
I suggest that the Gtk plugin copies the (style and language defintion) files from /usr/share/gtksourceview-4/ and the files /usr/share/locale/*/LC_MESSAGES/gtksourceview-4.mo when the library is copied.