Complex sentence: Today, through his work on etymology showing a close relation to Ryukyuan, it is more generally held that Japanese and Ryukyuan form a unique category together: that they are not strongly shown to be related to any other language group.
Split into simpler sentences: Today, through his work on etymology showing a close relation to Ryukyuan, it is more generally held that Japanese and Ryukyuan languages form a unique category together. That is, they are not strongly shown to be related to any other language group.