英語を母国語としない国の人が突然英語で話しかけられても普通に話すのを見ていつも感心する。
I always admire watching the person that a native language is not English speak normally even if spoken suddenly in English.
語順・単語等OKでしょうか？
I admire non-native speakers who can reply fluently when suddenly spoken to in English.
I'm always impressed when non-native speakers can reply in fluent English when suddenly spoken to in English.
「I always admire watching the person that a native language is not English speak normally even if spoken suddenly in English.」この文章が通じますが、もう少し自然な表現を教えてみたいと思います＾＾
I'm always impressed when non-native speakers can reply in fluent English when suddenly spoken to in English.
I admire non-native speakers who can reply fluently when suddenly spoken to in English.
「英語を母国語としない国の人」＝ non-native speakers (of English)
「突然英語で話しかけられ」＝ suddenly spoken to in English
fluently ＝ ぺらぺら、流暢に
admire/be impressed by ＝ 関心する
reply ＝ 答える、返事する、反応する
イギリス 
