I will outline three which I can remember for now-

THANK YOU/THANKS
Even though our aim of doing good shouldn't be to show off and enslave beneficiaries with incessant appreciation, we are happy and encouraged to continue our act of kindness when people respond to our good gestures by saying 'Thank You'.

Unfortunately many don't learn to say Thank You and many others think its enough to say it once. Some opine its only big gifts or notable acts of kindness that deserve repeated appreciation. Learn to say Thank You without weighing the good people do to you first, as long as its a kind act.

SORRY
This is a word many people mostly men do not want to say or say at the wrong time. The word Sorry is so powerful; it can unite nations and able to destroy empires. Sorry can sheathe sword and restore serenity. This word can build homes and also capable of razing homes to the foundation. Put aside your ego and say sorry when its needed.

I LOVE YOU
There is need to reassure our loved ones that we care and are there for them. Saying this to some people is enough to make their day. Learn to say I love you often to the right people.

Pls add other words/phrases you can think of

Also
PLEASE, when seeking a favour, help, or any other form of indulgence, and

EXCUSE ME when you are about to intrude on some ones space, conversation, time, e.t.c.

MAY I?
When you want to take or use something that someone else may be using, have in possession.

PLEASE
When you nead to indulge a person, a favour

EXCUSE ME
Before inconveniencing some one, intruding into their space, time etc.

Yes and No

Funjosh:
Yes and No

So true. Especially No.

Hi / How are you?
Great strangers that you seek information from first.

I do not know of other places in Africa but it is important to greet the door man or guard when visiting a place. You may just end up needing information from him/her and they take it personally when ignored.