Which plaintext languages to try?

[RadioFM]

IIRC in 13th or 14th century (Castilian) Spanish, 'j' is most often just a positional variant of 'i', primarily written after an 'i' and 't's (though not always) or at a wordbreak. There are however some few instances of 'j' and 'y' standing for semiconsonant /j/ around this time, and even more rare instances of an uppercase 'J' (sitting distinctly above the line level) used for /ʒ/, although this sound would most frequently be written with 'x', 'gg' or 'ggi' I believe.

As always, orthography was not standard and varied greatly depending on the period, place, and mood of the scribe.