I'll start looking into this

Change https://golang.org/cl/127551 mentions this issue: message: fixing documentation

I found a issue with your example, the correct fmt string is: Hoogte: %.1f meter\n.

I fixed the documentation.

@belimawr it's not only that unfortunately. What is the output? Comma should be used as decimal separator but dot is used instead. MatchLanguage is falling back to English. If I use language.Dutch it prints correctly but not with message.MatchLanguage("nl").

p := message.NewPrinter(message.MatchLanguage("nl"))
p.Printf("Hoogte: %.2f meter\n", 1244.9134) // Prints Hoogte: 1,244.91 meter

p = message.NewPrinter(language.Dutch)
p.Printf("Hoogte: %.2f meter\n", 1244.9134) // Prints Hoogte: 1.244,91 meter

OK, after reading the code and its tests it was more clear. We need to setup catalog otherwise it is always the default fallback language English. I think documentation should reflect this behavior. Following code works for English and Dutch with Dutch as the fallback language. It would be exteremly nice to have this in the sample code in documentation.

c := catalog.NewBuilder(catalog.Fallback(language.Dutch))
c.SetString(language.Dutch, "", "")
c.SetString(language.English, "", "")
message.DefaultCatalog = c

Edit: I think message.SetString(language.Dutch, "nl", "") also does the same thing without default fallback.

@kaskavalci I see your point. The thing is that the default catalog is empty and the fallback is English. I looked more at the code and documentation for catalog, and I'm not sure that the best method to get the language is message.MatchLanguage because it uses message.DefaultCatalog (that is empty by default) under the hood.

If the objective of that piece of documentation is to show how to use message.NewPrinter to get a printer that can print localised numbers, you could just use:

p := message.NewPrinter(language.Make("nl"))
p.Printf("Hoogte: %.1f meter\n", 1244.9) // Prints Hoogte: 1.244,9 meter

I created another issue #26767 before this investigation to improve the documentation, however now I'm not sure if it's a documentation issue, or a design issue.

Edit: let's keep this discussion on #26767 as it does not seem to be an implementation issue anymore.