This chapter describes a methodology for multilingual paraphrasing through mental image description language (Lmd) as the most essential working phase of our natural language understanding systems. Based on investigation of several languages, especially, English and Japanese, the author assumes that dependency grammar is applicable for grammatical description and processing of all the natural languages as well as Lmd representation for semantic description and processing, while morphological description and processing. In principle, multilingual operation system interprets source language text into Lmd expression in order to understand it and interprets the understanding result into target language text without using any syntactic information of the input. Multilingual operation system consists of five processing units, namely, source language syntax analyzer, source language meaning synthesizer, understanding processor, target language meaning synthesizer, and target language text generator. In the field of natural language processing, the syntax of a language can be provided so as to match its applications best.