Complex sentence: He also developed the Decode-Encode Language (DEL), which is documented in the early history of ARPANET, which, although never used, was an early precursor to Sun Microsystems's Java programming language.
Split into simpler sentences: He also developed the Decode-Encode Language (DEL), which is documented in the early history of ARPANET. Although never used, DEL was an early precursor to Sun Microsystems's Java programming language.