It's very hard to know, what substitution will be used. To do this, you should analyse every occurence in source code. As far as php variables are not hardly typified, this analysis connot be done automaticaly, as in case with greylisted strings.
And i think it's supposed, that translator can view strings in moodle interface and check in source code, how they works. I will never translate the string before i exactly know, where it will appear.