Did you ever get an answer on this?
I had to do something similar to support Italian and German translations and it seems to be working well.
In the file review.phtml in the directory app/code/frontend/mytheme/default/template/checkout/onepage/ (shew!) I replaced this line of code:
with this line:
<input type="image" src="<?php echo $this->getSkinUrl('images/btn_place_order.gif') ?>" onclick="review.save();" value="<?php echo $this->__('Place Order') ?>" />
<a class="img-btn btn-checkout" href="#" onclick="review.save();"><span><?php echo $this->__('Place Order') ;?></span></a>
So far so good. It’s behaving--translatable and correctly submitting orders.