Trouble with Simplified Chinese language pack

Hi guys, I'm having issues trying to install the Simplified Chinese language pack and I just don't know what to do - any help would be greatly appreciated.

I recently upgraded to v1.5.4 from v1.5.1 and everything went relatively smoothly. I decided it was time to make my site multilingual and installed the Chinese (Simplified) Language Pack. However, I cannot access my catalog when I switch to chinese, both on the site and in my admin. I thought maybe it was because of encoding differences so I ran the "Convert db2utf8" patch and changed from iso to utf in all relevant php files - to no effect.

Background/Info:
-Zen Cart version = v1.5.4 (manually upgraded from v1.5.1 ~2 weeks ago).
-Template = Ashley Pink.
-Plugins/addons = Edit Orders, Twitch Restrict Categories by Customer or Group (all latest versions).

I did everything on a test copy of my site, the address is http://test2.chineseherbs.co.nz/

Any help would be greatly appreciated, thank you in advance.

are you getting any errors in your /logs folder? you say you can't access when you switch, what happens? white screen? on screen error?

no errors in log folder. here are some screenies of what happens when i switch languages:

Before:
Attachment #15133

After:
Attachment #15134
(Clicking on ->(542) just refreshes to index page)

Before (admin):
Attachment #15135

After (admin):
Attachment #15136

In admin, I tried to populate the category fields for chinese to see if it would make a difference (in the eng version, clicking on edit on a category eg "Dried Herbs", then putting "Herbs" in the chinese textbox under category name) but after clicking save it would fail to save anything typed into the textboxes for chinese, whereas the textboxes for english worked normally eg I could change "Dried Herbs" to "ried Herbs" and it would save.

OK so after doing some more digging it seems like I've fixed the issue (for now). When I was looking around my db I found copies of my catalog with language_id=656 and language_id=827 (along with language_id=1 for eng), however there were no languages with these IDs, the language_id for zh/chinese was set to 3. When I changed this to 656 my categories/products/etc reappeared both on the site and in admin and everything seemed to work normally again.

I'm guessing the extra language_id's were artifacts from my previous attempts to install the chinese language pack, however im still puzzled over why the IDs were set to 656 and 827 (instead of say 2 or 3) and whether this 'fix' really works...