English is the outlier: Spanish hueso and Esperanto osto both trace to PIE *h₃ost-* — kin to Greek osteon and Sanskrit asthi — while English reached into a Germanic drawer the rest of Europe never opened. The German cognate survived but shifted: Bein now means leg, the skeleton-word reborn as a limb just across the Rhine. Chinese 骨 arrives from wholly different territory, a Sino-Tibetan root in a character that refuses to show bone without the flesh around it. Zamenhof, building without inheritance, chose Latin's bare stem: osto — structure, no tissue.
Closest cognates are Dutch been and Old Norse bein. German Bein, the direct cognate, shifted to mean 'leg' in modern usage; Knochen is now the standard German word for bone, leaving Bein's original skeletal sense only in compounds.
Regular Spanish diphthongization: Latin short o before a single consonant became ue- (cf. porta → puerta, focus → fuego). The Latin genitive stem ossis supplies the oss- visible in osteología and óseo.
A bare joint (冎) seated above a wedge of flesh (月 standing in for 肉): the character insists that bone must be perceived within its living surround. You cannot write the hard thing without writing the soft thing around it.
The 月 radical in body-part characters (脸, 肺, 腿, 骨) is a reduction of 肉 flesh, distinct from 月 meaning moon or month. 骨 extends figuratively to moral character: 骨气 (gǔqì) means backbone in the sense of integrity or resolve.
The collective suffix -aro yields ostaro (a full set of bones, a skeleton's worth). The root ost- recurs in scientific borrowings: osteoporozo, osteologio. The -o suffix is the standard Esperanto noun ending, contributing no lexical meaning.
Every language buries its dead differently — but all of them kept the bones.