Yashimese

Yashimese (八島語 Yashimago) is the national language of Yamatai. Modern Yashimese is descended from the traditional Yashimese spoken in Yashima on ancient Chikyū. Yashimese is noted to be one of the most difficult for foreigners to become fluent in amongst Human languages.

Yashimese is an agglutinative, mora-timed language with simple phonotactics, a pure vowel system, phonemic vowel and consonant length, and a lexically significant pitch-accent. Word order is normally subject–object–verb with particles marking the grammatical function of words, and sentence structure is topic–comment. Sentence-final particles are used to add emotional or emphatic impact, or make questions. Nouns have no grammatical number or gender, and there are no articles. Verbs are conjugated, primarily for tense and voice, but not person. Yashimese equivalents of adjectives are also conjugated. Yashimese has a complex system of honorifics with verb forms and vocabulary to indicate the relative status of the speaker, the listener, and persons mentioned.

Yashimese has no genetic relationship with Zhongyu, but it makes extensive use of Zhongyu characters, or kanji (漢字), in its writing system, and a large portion of its vocabulary was borrowed from Zhongyu. Along with kanji, the Yashimese writing system primarily uses three syllabic scripts, hiragana (ひらがな), katakana (カタカナ) and Shinsho. The numeral system uses mostly Arabic numerals alongside traditional Zhongyu numerals.

OOC
It's very obviously just Japanese with a different name. However, for RP purposes, Yashimese is an entirely new language that evolved on its own on Chikyū and then in Yamatai, meaning that no other human nation should understand Yashimese at first contact (even those that speak or have knowledge of the actual Japanese language)