Im upgrading to 10, and want to change some of the system terminology, for example 'Blog' to 'Newsfeed', 'Friends' to 'Colleagues' etc.
The exhaustive default list is, in fact, only those 26 items. Site terms are unfortunately a somewhat legacy feature that are difficult to extend for multiple cultures and rarely consumed by the UI. While you may find changing them useful, I'd suggest you still investigate resource string exports/imports, as these phrases will likely appear more often than not directly within resource strings rather than as term references.
That said, here is the default set:
It's in Administration > Translation > Terms.
In addition to site terms, you may find it helpful to adjust the resource strings in your themes and widgets as well, as many do not always consume the terms defined in Administration > Translation > Terms.
Resources can be safely customized individually directly in themes and widgets in Widget Studio and Theme Studio. Or if you want to do many at once, those resources can also be exported en masse to XML files, adjusted, and re-imported.
For widgets, it's in Administration > Interface > Widgets > (three dots icon) > Export All Resources. Subsequently, an adjusted resource file can be re-applied with in Administration > Interface > Widgets > (upload icon).
If you are on 10.2 or later, it's the same process in Theme Studio. (Administration > Interface > Themes).
Each widget and theme defines all of its own language resources that it needs for all of its text. Those can be customized and/or translated both individually and en masse. I provided a bit more detail in my other reply to this thread. Please let me know if you have any questions!