In which of the following texts, it is stated that those who could not speak…

Concept: In Vedic literature, 'Mleccha' could function as a linguistic label — describing speech judged incorrect or unintelligible relative to the correct, ritually-prescribed Sanskrit form. In this usage the label is grounded in speech-correctness rather than in a person's ethnicity or region.

Application: The Shatapatha Brahmana, a prose text of the Shukla Yajurveda tradition, preserves this specific episode: defeated by the gods, the Asuras' cry “he 'rayah” (O enemies!) came out mispronounced as “he 'lavo”, and the text states that such faulty utterance is Mleccha speech — hence a Brahmin should not speak it. Here the label Mleccha rests on correct versus incorrect Sanskrit articulation.

Cross-check — content of the other candidate texts:

• Shvetashvatara Upanishad is a theistic treatise built on Samkhya-Yoga concepts; it opens with enquiries into the cause of the universe and develops Rudra as identical with the universal Brahman, and uses the simile of a chariot yoked to unruly horses to illustrate controlling the mind through yogic practice.

• Gopatha Brahmana, structured in a Purva-bhaga and an Uttara-bhaga, is the sole surviving Brahmana of the Atharvaveda; it lays out the mahavrata rite and other Atharvavedic ceremonial procedures, and enumerates the six Vedangas.

• Brihadaranyaka Upanishad contains the “neti neti” (not this, not this) formulation for describing the imperishable Atman, and records Yajnavalkya's dialogue with his wife Maitreyi on why wealth cannot grant true immortality.

Result: The episode that grounds the term Mleccha in incorrect Sanskrit articulation is found in the Shatapatha Brahmana.