Include is a compile time directive, you can't use actionscript (which runs after compilation) to affect it.
However, you can use conditional compilation. I can imagine this making sense when you have tons of files at different places and wat to be able to switch between them.
You can define compile time constants in Publish Settings -> ActionScript 3.0 Settings -> Config Constants -> Add as much constants as you need, in this case:
CONFIG::ENGLISH true
CONFIG::GERMAN false
CONFIG::ENGLISH function init() { include "en/file.as"; }
CONFIG::GERMAN function init() { include "de/file.as"; }
init();
Result: "en/file.as" gets included.
Conditional compilation enables you to define multiple function or class definitions and with setting the Config constant, you get different results.