A feature that - most often - is quite helpful is string substitution/string formating. For example one of the following commands:
"Hi my name is %s and I'm %s years old" % ("Derp", 12)
or
"Hi my name is {0} and I'm {1} years old".format("Derp", 12)
will be evaluated to
Hi my name is Derp and I'm 12 years old
in python. Is there a (predefined) way to do something similar in Haxe?
Yup.
It's called String Interpolation in Haxe.