Literal Translation: His mouth is honey; his heart is a sword.
This kind of metaphor would be very confusing for foreigners. If you want to say some one is “口蜜腹剑，笑里藏刀”, the better expression is “with honey on one’s lip and murder in one’s heart” or “honey-mouthed but dagger-hearted”. A more direct translation is: He is an evil man who has a mouth that praises and a hand that kills.
Someone translate it into: If you neglect study when you are young, what may happen to your old age?
This would be extremely confusing for foreigners as they do not know the concept that being good at study means being successful, neither do they think that you would be misterable if you are not good at study. They tend to say “Studying / working hard can lead you to a brighter / more promising future. (努力用功会带给你光明的前程。)” when they want to encourage their children to work hard.
Surely we can say this to young people:
Study hard when you are young. It will pay off when you get older. (年轻时好好努力，到老的时就会觉得学有所值。)
If you literally translate it into:
Learning is like rowing upstream; not to advance is to drop back.
It is not very clear for foreigners to understand. We can revise it to:
Learning seems like rowing upstream(逆流); if one does not advance, one will fall back. Learning is like rowing against the current(激流), if one does not advance, one will retreat. A more direct saying is:
If you don’t make progress, you will fall behind.
There are several translations:
He who teaches me for one day is my father for life.
If you are my teacher for even one day, you will be my teacher (mentor) all my life.
He who teaches me may be considered my father-figure for life.
Those translation all express a meaning that “老师即使对我只有一天的教诲，也会让我一生都受用不尽”
Threre are dioms with similiar meanings in English, such as:
Teach others to fish and they will fish for a lifetime.(授人以鱼，不如授人以渔)
Give a man fish, he will have a meal; teach him to fish, he will have food all his life.
One translations is:
Faithful words are contrary to the ears; good medicine tastes bitter to the mouth.
Translate it from another direction:
Bitter words are medicine; sweet words bring illness.
Good advice often jars on the ears; bitter pills have good effects.
All those translations are not as good as:
Honest advice may be distasteful to the recipient.
Honest advice is hard to accept.
Truth is a hard pill to swallow.
More simpler saying is:
one who stays near vermilion gets stained red, and one who stays near ink gets stained black.(vermilion 是朱砂或鲜红色的意思)
When you touch black, you become black; when you touch red, you become red.
More familiar sayings in English are:
One takes the behavior of one’s company. (一个人的行为，往往受到朋友的影响。)
One takes on the attributes of one’s associates. (attributes 复数時是指品质、特性；associates 指朋友、同事)
To put it simply:
Watch the company you keep!(小心交朋友！)
Or you can say it like this:
As he who lies with dogs will ride with fleas, how could your son learn anything good by mixing with those people? (你儿子跟那些人混在一起,能学出个好来?)