This is not about physics, it's about translating an idiomatic expression. Even in Germany, where people don't even remember how much a mile once was, seven-league boots are still Siebenmeilenstiefel (literally seven-mile boots) and people go meilenweit (for miles). And we are divided between losing a few pounds (a pound being a metric pound equal to 1/2 kg) and losing a few kilos - depending not on how much we want to lose but on the speaker's age and socialisation.
The idea that idiomatic expressions have to change immediately with metrication is just a canard spread by anti-metrication activists.
And for us Europeans kilos is correct. And what, pray tell, is wrong with metric measurements? Better than the length of some dead king's foot or arm!! If I said to any of my French speaking Swiss friends I am going to loose a few pounds they would think I had taken a FOREX contract out on sterling and bet wrong!
The "king's foot" was also never based on the actual anatomy of a king (in a republic it would have been called the "national foot"), but it seems you just want to be polemic. It's utterly ridiculous that the UK is still not fully metricated, and the situation is even more ridiculous in the US. But a fundamentalist attitude is not going to help.
Idiomatic expressions just don't immediately adapt to new circumstances. In German we even still say "Wer den Pfennig nicht ehrt, ist den Taler nicht wert." The Pfennig (subdivision of the Mark) was abolished only a few years ago, but there is not a single person alive who remembers paying with the Taler. Yet our version of "Take care of the pennies and the pounds will take care of themselves" still features it.
Here it's the same thing. The sentence is clearly not a mathematical statement to the effect that someone wants to lose x kilogrammes, where x is a positive integer. The unit just doesn't matter as long as the order of magnitude is appropriate for measuring human weight. Just like it is sometimes best to translate a singular by a plural or make other, similar changes, here it's best to replace kilos by pounds unless there is a special context that requires kilos. (Such as the next sentence referring to the kilos with a specific number: "I already lost one".)
Canada is very confused. Has been for 40 years. We use a more or less random mix of Imperial and Metric. Pierre Elliot Trudeau's vision of bilingualism has ingloriously failed. But, we can all fly at 33,000 feet doing 850 kph and buy a twenty-six ouncer of scotch while 400 miles from our European holiday destination above the chilly (6 degrees C) North Atlantic,. which is running about 2 metre swells today.
Note: I'm never sure if the twent-six ouncer is American fluid ounces or British....agghh.
I disagree. But I acknowledge I'm a stickler for the "hierarchy" of quantitative pronouns. Two=couple<few (three)<several<some<many. If you look closely at their definitions you'll realize this is true despite their overlap and the laxity with which many (definitely more than some) people use them.
United States, Virginia. If I referred to three things casually as "a couple", very few people would criticize. So yes the literal translation would be "couple" = 2 things but "a couple pounds" is synonymous with "a few pounds" as the speaker isn't literally saying they need to lose exactly two pounds (unless they are aiming for a very specific weight).
Translation from English to French is a red herring. The important point is that "a couple of kilos" and "a couple of pounds" are perfectly correct and natural translations of "quelques kilos" when translating from French to English.
See definition 3 here: https://en.oxforddictionaries.com/definition/couple . It's marked informal, but the present French sentence is informal as well.
In my experience, most native speakers of English use "a couple of" synonymously with "some", but some of them deny that they are doing it. Example: I asked a bus driver in Leeds how long it was to a certain stop. (The precise number of minutes was critically important to me because I had to decide which direction to go to get a connection.) He said: "a couple of minutes". When I asked what that is in more precise terms, he insisted it meant "two minutes" and implied that I was stupid to ask. It took ten and I lost my connection.
By the way, exactly the same phenomenon exists in German, except we capitalise "Paar" when it really means two and don't capitalise it when it means "some". Consequently, nobody denies that both meanings exist.