Does a concealed Kan interrupt the game flow?
Warning: not exactly familiar with the term Riichi, nor with the specific Mahjong ruleset Riichi Mahjong (Henceforth, RMJ). 

In the books I've read, and the 3 different published rulesets I've played (One of which was a 1960's Japanese rulebook translation), the functional definition of "on the first turn" is "without having discarded."

Having just read the rules for Riichi from the EMJA, the rules for Riichi Blessing of Heaven (East out on deal), Blessing of Earth (non-east out on self-drawn tile), and Bleassing of Man (non-east out on claimed discard) forbid concealed Kong.

Both the RMJ rules and the 1 of the rulesets I've played agree that any play after a discard is claimed is no longer first turn. 

TLDR: in the Riichi ruleset, yes, Kans/Kongs do preclude the "first turn" bonuses.