I am no expert, but since nobody else has replied, I will try.
I experimented with the German language pack and displayed a Diagram chart.
It translated Born, Marr & Died correctly even though there are no translations for those explicit words in the language pack.
So it looks like the translation for Birth also handles Born, and likewise for Marriage/Marr and Death/Died.
So try Tools > Preferences > International with Default Output Language: Slovene and produced a Diagram chart.
It may already handle those translations.
Alternatively, in the ‘Edit Translation Tables for …’ dialogue have you tried the New… button to Create New translations?
That seems to be the way to add missing word translations.
I suggest you use the Help buttons in these dialogues to obtain pages of advice.