Are we looking at Hittite, Middle Hittite, Neo-Hittite or Old Hittite? The ISO 639-3 language code list has all four of those. Phrygian only has the one entry.
See
ISO 639 Code Tables for the complete searchable list.
Wrapping an string in the language code should prevent a spellchecker from attempting to spellcheck it unless it has a matching dictionary. Something like <span xml:lang="xpg">*k’ṃtom</span> for example.