the site about wastewater treatment systems in thailand which you mentioned is not using cakephp's standard translatable feature. You might have seen that translatable is broken in SS 2.3.1 (which that site is using). That site is using the alternative multilingual method (discussed also here) which works fine with 2.3.1.
The ecommerce version used is NOT the stable module, but it was from trunk, also the payment module, and required some corrections.
it is a little difficult to help you because you are not saying clear enough what you do.
- Which SS version?
- Which ecommerce version?
- Which method for multilingual content?
The code used in that site is not using the language switcher widget or so, but a simple piece of code I wrote. If you need it I can post it. But it will not help you if you're not using the alternative multilingual method.
Keep in mind that with SS 2.3.2 translatable is fixed and better than before, so you do not need the alternative multilingual method any more if you use 2.3.2, which is available as beta or rc already. I recommend to use that, as it is the official SS way to do multilingual and works again. Dunno however if ecommerce is working with 2.3.2 already, I assume you would need the latest ecommerce and payment code from trunk.