As far as I can tell Fluent has no builtin support for tagging a phonemic information to messages or arguments (it only supports plural rules and number formatting largely derived from the CLDR). You can probably specify special cases with selectors (but it will quickly go absurd, for Polish I guess that applies to 6, 7, 16, 17, 60..79, 100..199, 600..699 and so on?) or have an external function.

Context: Polish preposition "z(e)" is spelled "z" if the following word starts with s and z and alikes (problematically enough, the exact rule is not systematic) and "ze" otherwise. Korean has a similar case with postpositions "은(는)" and "이(가)" where the former is for words ending with a consonant and the latter is for words ending with a vowel, and the ko-KR localization of Firefox seems to completely ignore and/or sidestep this; the last letter is assumed (e.g. "{-brand-name}는") or a static word is inserted (e.g. "{$user} 사용자는" instead of "{$user}은(는)").