Alec Smecher writes that...
The only critical files that need translation are the locale/en_US/locale.xml and
The first contains all the strings used in the user interface; the second contains all the system's default emails. (The latter can also be translated from within the system, but it's better to translate the XML file instead.)
To create a Turkish translation of the locale/en_US/locale.xml file, make a new directory called locale/tr_TR, copy the locale/en_US/locale.xml file into locale/tr_TR, edit it, and translate. There are currently 1865 translation keys in this file (approx. 10,000 words) that need to be translated.
Follow the same process to translate the email templates; there are 43 of these templates in the XML file mentioned above (approx. 2,500 words).
Once these are finished, add a new entry to registry/locales.xml. Afterwards, you can go in through the Site Administrator interface and install the translation.
There are several other files which you might consider translating, but they're not necessary. You can follow the same steps to translate each of them. They are:
* Help files in help/en_US/toc and help/en_US/topic; I wouldn't suggest translating these, as they're likely to be heavily updated in the near future.
* Reading Tools files in rt/en_US; these files describe a set of search engines made available to readers when browsing journal articles. I suggest looking through the English version of the Reading Tools and potentially translating whichever version is most appropriate.
* Plugins locale information in plugins/*/*/locale; these are very small files and easy to translate. Currently, they contain all the user interface strings used by the import/export plugins. In the future more types of plugins will be available.