Ok, great, thanks for the detailed explanation, so if im understanding correctly, the way i envision the locale stuff to work should be fine for you, so you would have something like:
<dialog id="dialog1" title="_(DIALOG1_STEPPER_TITLE)">
<hbox>
<label id="label1" text="_(LABEL1_STEPPER_TEXT)" verticalAlign="center" />
<number-stepper pos="100" />
</hbox>
</dialog>
And those strings would be inside some .properties file or other format… when the locale changes, or is initially found then those strings would “magically” update themselves. This wouldnt be dissimilar to how the current binding works, outlined here: Better Binding in HaxeUI - although this would be a more specialized version of it
A minor thing is im not sure about the “_(…)” syntax - im not sure i like it - but i guess is not really important
I think thats enough info to start a first iteration, but let me know if you have any more thoughts / ideas… always useful to get as much input as possible when creating a new core sub system 
Cheers,
Ian