Is it common to say 'Kilo' to mean 'Kilogram'?
Solution 1:
Kilo as a short form of kilogram dates back to at least 1870, according to the OED, long before Britain adopted the metric system. You can find it in American dictionaries as well; AHD simply puts it as
ki·lo n. pl. ki·los A kilogram.
and Merriam-Webster consider this its primary definition.
The OED also notes that kilo can be short for kilometre; however, in current usage, this is unlikely to be the assumed meaning outside of narrow contexts. If nothing else, kilo has been reinforced among the public as a unit of mass due to its common usage in television police shows involving drug smuggling.
In more formal communications, I would still advise spelling out kilograms (or using kg) to avoid any ambiguity, however.
Solution 2:
Yes, Americans know the word kilo. However, this word is not meaningful to them the way it is to you. This is because they don’t have any common reference points that would allow them to immediately understand how much something is in kilos. If you told them something was 5 kilos, they couldn't tell you whether that was like a bag of sweets or a tunafish sandwich or a gallon of milk or a bag of potatoes or can of gas or their mother-in-law. Since nothing is measured in that here, it is just a random foreign number. It would be like telling you that something was 5 acres or 5 feet or 5 ounces: you'd know the word, but the quantity would not be something you could associate with anything concrete in your own life.
So no matter whether you said kilos or kilograms, many Americans would blank if you used these units in your conversations with them. It isn't a unit of measurement that is used here. It's a foreign language.
Some might think when you said kilos, you were talking about kilometers not kilograms, and many would be unable to fathom how much whatever you were talking about really was even if they figured out that you meant mass-qua-weight. They have nothing to compare it with so it’s just random meaningless numbers to them.
You should therefore always convert to American measurements when talking to Americans, just as you would convert Serbian to English. That means you should convert kilograms to pounds.
This does not apply to technical conversations with scientists accustomed to using these units. But the overwhelming majority of Americans will have no idea what you are talking about, just as though you were talking to them in Serbian instead of in English.