The Welsh or Icelandic "ll" is not quite the same. That's a "voiceless lateral fricative", lacking the alveolar break that earned it the "t" in "tl" for the Latinized spelling. It's much closer than most languages get, but it is a different sound.
The Nahuatl consonant is a "voiceless alveolar lateral affricate". It is a single constant represented with [tɬ] or, more correctly, with a tie bar between those two glyphs: [t͡ɬ].
The Welsh or Icelandic "ll" is not quite the same. That's a "voiceless lateral fricative", lacking the alveolar break that earned it the "t" in "tl" for the Latinized spelling. It's much closer than most languages get, but it is a different sound.
The Nahuatl consonant is a "voiceless alveolar lateral affricate". It is a single constant represented with [tɬ] or, more correctly, with a tie bar between those two glyphs: [t͡ɬ].