uncle cheese asked me to explain how i got dataobjects working with translatable, see http://www.silverstripe.org/dataobjectmanager-module-forum/show/260986?start=24#post267112
At the moment translatable doesn't work, cause even ComplexTableField, Dataobjectmanager extends, cannot handle locale. There is a patch http://open.silverstripe.org/ticket/4435 which adds the locale parameter to all urls of CTF, i made another for passing locale with DOM, see http://open.silverstripe.org/ticket/4492
This for now fixes the "I can't handle sub-URLs of a CMSMain object" Bug.
So far so good.
I haven't used image gallery yet, so i don't know what you did. I'm also pretty new to Silverstripe but may have some php skills.
In my project i have used translation decorator ONLY for the pages, not for the dataobjects. The way translation currently works, is, that it makes a complete translation of ALL items, so a completely new item. Not so good for image galleries.
A (maybe hackish) approach would be, to add e.g. title_en, title_de, title_nl to each picture and make a custom getter which returns title depending on the current locale. So you only have some fields translated but not the picture.
hope that helps a little bit - i sometimes hang around in IRC #silverstripe if you prefer real time discussion ;)
in my current project i have a many-to-many relationship between translated page and dataobjects. so i easily have them available in all pages and translations. see my later post on translation groups for a possible way in has_one relationships