If you want to help translate:
a.) i hope crowdin would be ready soon. Will make things a lot easier.
b.) at the moment you can translate it manually and export it.
It's very simple...
Step 1.) If Language doesn't exist: Go to "Localization --> Languages" and Add your Language. Make Sure the Settings match PrestaShop language settings (Example Image German)
Step 2.) Go to "Localization --> Translations -->modify translations" and translate the Frontend, Backend, Errors, Modules and PDF.
You also can change Translations of Mails inside PrestaShop. But i prefer to do this directly with Editor like Netbeans, notepad++ or other...
Step3.) If you get a message like that:
Warning! Your PHP configuration limits the maximum number of fields allowed in a form:
1000 for maxinputvars
change the values "maxinputvars" and "maxpostvars" in php.ini (8000 should be high enough".
Step 4.)
After translating everything you can export it.
Go to "Localization --> Translation --> Export a language". Choose your language and Community theme default and hit the export button.
you will get a gzip file including backend, mails, pdf, modules, templates translations.
Step5.) Upload your gzip in this forum Post --> Done!
PS: You can't translate the admin Menu (Admin Tabs).
Translations of back office tabs are stored in datbase, check tables: pstab, pstab_lang