There a sizable difference between the 2nd edition ruleset and 5e. In 2e, used by BG1 and 2, spells had a casting time, measured in segments, during which they can be interrupted. This was what the incantations represented. In 5e rules, spells have no casting time, so no time for incantations.
That's why I propose to implement It diffently, as described in my first post: use similar memorable phrases that were used as incantations in previous installments but in a bit different fashion - once spell selected, but BEFORE cast. Kinda like your character recalls the spell by quietly ppronouncing phrases, before actually casting it.
