I'd love to hear you spit in Croatian man! I think rapping in English is overrated honestly, I try and mix it up sometimes with the bit of Malay and French I can speak.. About the tightening of the flow, mainly it's the way your syllables syncopate with the beat, now this could be due to the factors you mentioned above, but I often hear it with emcees who don't rap in their mother tongue.. get what I mean?