There are many Japanese phrases which are not able to translate to English.
But if you can say the following words/sentences, your client would be happy.
“Osewani-narimasu” (お世話になります) :
This is difficult to translate. “Osewa” means “taking care” “ni” is connection word, “Narimasu” is being.
Japanese people tend to start email from this word. Or, we use this words to client when we start business meeting.
If you can say “Osewani-narimasu” at your business scene like business meeting, telephone, email, your client would be surprised!
This is much more difficult than “Osewani-narimasu” to explain.
“Yoroshiku” means like “Doing something smoothly”. “Onegai” is “please” and “Shimasu” is added for politeness.
“Yoroshiku Onegai Shimasu ” is very useful to end email message, or phrase when you finish the meeting.
“Otsukare-sama-desu” means “You are tired” if we translate directly, but it is like “say hello” to your colleagues.
When you meet your colleagues at the office (when your colleague return to office), or when you make/take a phone call, you can use this word.