の母語です

is someone's native language

→ の
phrase Rare #20,418
is someone's native language 母語 means “native language” or “mother tongue.” The の marks possession, and です is the polite form of “is”; the phrase can be made casual with だ or の.
JA: 英語は彼の母語です。
EN: English is his native language.

フランス語が彼らの母語です

French is their native language.

スペイン語が彼女の母語です

Spanish is her native language.

フランス語がトムの母語です

French is Tom's native language.

イタリア語の母語です

Italian is my native language.

ヘブライ語の母語です

Hebrew is my native language.

フランス語はトムの母語です

French is Tom's native language.

ポルトガル語の母語です

Portuguese is my native language.