Which plaintext languages to try?

[nablator]

I've checked a few cipher tables from Tranchedino's collection, they seem to only use I and either V or U.

No need for "j" and "v" so ciphers didn't have symbols for them; they didn't exist as separate letters, only as writing conventions: "j" was only used in manuscripts and printed books for the last of a sequence of "i" (for Roman numerals and Latin "ii" endings), a very consistent convention. Uppercase "u" was written V, never U (because the Romans wrote it like that, and Romans knew better). They started replacing all word-initial "u" with "v" (including vl..., vt..., vp...) during the 15th century, not in all manuscripts, some had only "u" as before. The typographic convention of initial "v" remained until the 18th century (maybe a little earlier, ca. 1650, I don't know exactly), when the standard changed again and became phonetic: the choice between "i/j" and "u/v" was meant to differentiate vowels and consonants. This is for Latin and French, I don't know about other languages.