YOU SAID:

We must speak in mattp's language when addressing him from now on.

INTO JAPANESE

これから彼に話しかけるときは、mattp の言語で話さなければなりません。

BACK INTO ENGLISH

From now on, when you talk to him, you must speak in mattp's language.

INTO JAPANESE

今後、彼と話すときは、mattp の言葉で話さなければなりません。

BACK INTO ENGLISH

From now on, when I talk to him, I must speak in the language of mattp.

INTO JAPANESE

今後、彼と話すときは、必ず mattp の言語で話さなければなりません。

BACK INTO ENGLISH

From now on, whenever I talk to him, I must speak in mattp's language.

INTO JAPANESE

これからは、彼と話すときはいつでも、mattp の言語で話さなければなりません。

BACK INTO ENGLISH

From now on, whenever I talk to him, I have to speak in mattp's language.

INTO JAPANESE

これからは、彼と話すときはいつでも、mattp の言語で話さなければなりません。

BACK INTO ENGLISH

From now on, whenever I talk to him, I have to speak in mattp's language.