Factorio Forums

Localised strings appear to not only consist of strings, bools or arrays of itself.
Apparently they can also be numbers (integers and floats, tho double would make the most sense here I guess).

The provided examples even show localised strings with numbers in them

They indeed allow numbers. Added to the docs for the next version, thanks for pointing it out.

Note that in 2.0, they do not allow any number or bool - you need to call tostring() on them first if you want to use those types.