The translations aren't supposed to be added to CB Activity unless you're not using CB Activity 3.0.0. Older releases have their own timeago usage and would need their own translation, but with 3.0.0 it's just using CBs built in timeago which is the same timeago being used by the news display. See my below screenshots.
Language Strings:
Backend News Module:
Frontend CB Activity:
These strings are not CB Activity specific. The translation does not go into CB Activity language files. It goes into the core CB language file as shown above in my first screenshot. In the case of the English language plugin that would be the below location. They will already exist in the below file and just need to be translated as needed.
components/com_comprofiler/plugin/language/en-gb/language.php
krileon
