Dear Dominik, Not just Kaiyaṭa but also other commentators take mleccha of mleccho ha vā eṣa yad apaśabdaḥ to be a derivate with ghañ signifying a karman.  And clearly they mean to derive the term from mlech with ghañ in accordance with akartari ca kārake sañjñāyām.  They also derive mleccha of mlecchā mā bhūma from the same base, but with agentive ac (nandigrahipacādibhyo lyuṇinyacaḥ).  Thus, mleccha refers first to a term (śabda) that is indistinctly/incorrectly uttered, then to the speakers who utter such indistinct terms.  Patañjali explains that apaśabda refers to what is commonly known as a mleccha.  That is, emphasis here is on incorrect speech (apaśabda) in the phonic aspect, identified with the indistinct speech of the Mleccha ('barbarian' if you will), as is clear from the Śatapathabrāhmaṇa passage comparable to the present Bhāṣya passage. I think The Sūktiratnākara and others got this right.  The contested issue of the syntax is, to my mind. not so clear as some have made it.