There are more than one way to express this sentence in Chinese
1.
"My co-workers talk among themselves" = "我的同事們私下交談"
"My co-workers (use English) (when) they talk among themselves" = "我的同事們私下交談(時)(使用英語)"
"我的同事們私下交談時(都是)使用英語" - Add 都是 (always) to make it more clear
2.
"My co-workers talk among themselves" = "我的同事們私下交談"
"(when) my co-workers talk among themselves, (they use English) " = "(當)我的同事們私下交談(時), (他們使用英語)"
"(當)我的同事們私下交談(時), (他們都是使用英語)" - Add 都是 (always) to make it more clear
Edit:
dan wrote:
私下交談 denotes those informal circumstances, which doesn't well address the sense of talk among themselves.
I replied:
'私下交談' means 'talk privately' implying 'without including people outside of their circle. May be I can change '我的同事們私下交談' to '我的同事們在他們之間交談' to make the translation more direct, but that would be quite wordy
If I decided to use the more direct translation, the sentences would become:
"我的同事們(在他們之間交談時)都是使用英語"
"(當)我的同事們(在他們之間交談時), 他們都是使用英語"
But it can be rephrased to make less wordy:
"我的同事們之間的交談都是用英語" (the conversations between my co-workers are all in English)
amongst
could just simply be omitted here without losing clarity.