Yes, for l10n handling I'm aware of the trade off, gspell on GitHub will need to use something else (weblate for example), or in the meantime just with traditional methods for contributing *.po files.
About GtkSourceView and gedit, I just continue the development of those projects where I've left them. First, for my own needs, and at my own pace (I don't follow the 6-months GNOME schedule). But the development is done in the open. I could make those projects parallel-installable with the versions from gnome.org, so that distributors can package them. Similar to the Unix command cc with several implementations available (gcc, clang), there could be several implementations for the gedit text editor. Linux distributions even have a system for the user to choose which alternative is best for his/her needs, with update-alternatives.