まともに会話できない人といくら話しても無駄だ！って英語でなんて言うの？

「まともに会話できない人」＝ someone who is bad at (making) conversations/a bad conversationalist/someone useless at holding a conversation 「無駄だ」＝ a waste of time/pointless 「いくら話しても」＝ it doesn't matter how much or what you say 組み合わせると： It doesn't matter how much or what you say because talking with someone who is useless at holding a conversation is a waste of time. 又は It's pointless trying to talk to someone who is a bad conversationalist

「まともに会話できない」に関連する”「聞く耳を持たない」「分からず屋」や「頑固」”などの英単語では、rigid/hardheaded/stubborn/inconvincible/narrow-minded など、たくさんあります。