If you're really interested in the language, the Yonaguni Ethnographic Museum sells a small dictionary written by an island auntie who is the museum's curator, Nae Ikema. The same woman, 87 years old, is also the last remaining soul with a traditional knowledge of the island's peculiar writing system known as "kaida dii" (two symbols found carved in the underwater ruins closely resemble the local characters for horse and goat, respectively).
In general, Yonaguni only uses the 3 vowels, a, i, and u (compared to Okinawan, with 5, Yaeyaman, with 4, Miyako, with 4, and Amami, with 8), but e and o are still heard occasionally, such as in the imperative hire: (go!) and the emphatic particle do:.
The first thing that a speaker of Common Japanese is likely to notice is that initial y sounds (IPA [j]) have changed into d sounds, such as in du:ci ("four"; Okinawan yu:ci; CJ yottsu), dumi ("wife"), and Dunan, the island's name. Intervocalic voicing is also heard -- CJ haka (grave) is Yonagunian haga; CJ aka (red) > Yonaguni aga.
Aspirated t and k sounds, often indicated with an outward-facing apostrophe (ʻ), are distinguished from their unaspirated counterparts, which can be indicated with an inward-facing one ('). Vowel length varies with the speaker, and when writing in katakana some will use the long vowel mark and some not.
c - like English "ch"
ŋ - like the "ng" in "ing"
ʻ - indicates that the preceding consonant is aspirated
Many Yonagunians who moved to the island as adults can't speak the dialect fluently and say that the only thing they can do is count to ten. If you can master this, you'll get a good summary of the sound changes in Yonagunian in the process (standard Japanese in parentheses):