noun
cognate language
Linguistics term for a language that shares a common ancestor with another language.
日本語と韓国語が同系語であるかどうかは議論が続いている。
Whether Japanese and Korean are cognate languages is still debated.
語族 is 'language family', the broader group; 同系語 are individual languages within that family.
Compound of 同系 (same system) and 語 (language).