It's common here to say something like "this week it's in the 70s." In places that use Celsius, "in the 20s" must be too broad, so do they instead say something like "high 20s"?
Here in a metric country (Australia) they use words like cold, cool, warm, hot.
Otherwise people turn to BOM (Bureau of Meteorology) hour by hour / day by day records and forecasts of temp. and rain with radar cloud coverage - these just use nearest whole Celsius numbers, eg: https://www.bom.gov.au/location/australia/western-australia/...
Almost everything you've conjectured about "how they do it in a metric country" has been wrong; builders don't use a third of a metre, for example, everything is typically in millimetres - 1200, 1600, etc (aka, 1.2 metres, 1.6 metres) along side usages of 1 mm and 5 mm for small tolerances, expansion gaps, etc.
They don't use a third of a meter because, like you said, you can just use some number of mm or cm that is divisible by 3. We don't use decimal inches because fractions are easy too.
Why would you assume that this particular turn of phrase would be common at all?
People usually just say the temperature they’ve heard in the weather report, without giving a range. If you say “over 20”, people don’t assume “close to 30”, because then you would have said that.
Because it's really common here. This week it's in the 90s, which is very hot. The temperature changes day to day, so how do you give the approximate temperature for the week?
You’d just say “over 30” for that kind of temperature, or “almost 40” if it’s at the higher end. I think you’re taking your own unfamiliarity and just assuming that we’re all struggling because you don’t know how to translate what you’re used to, but I promise you nobody who is used to it is finding it inconvenient or difficult to use.
