Set the locale via Magento admin area and create translate.csv in app/design/frontend/<package>/<theme>/locale/languagecode_COUNTRYCODE
You can access and download the file via FTPmanager or Hosting Control panel File manager.
The goal is to put translated strings inside the translate.csv file. Some templates may already contain the locales for some languages.
Important: before translating, you need to note that all strings should start and end with double quotes and should be separated with comma (,), not a semicolon or any other character. For instance:
"Default line","Translated line"
Text United makes it very simple to translate a translate.csv.
Start by creating a new translation project. From your dashboard click New project icon and then select the second option (2.) to upload your file.
Follow the setup process for the translation project and enter the language(s) you want your file to be translated to (the system will generate separate csv files, one for each language after translation).
Once translation is done, import the translated file back to your Magento in the corresponding locale folder.
Remember to refresh all caches after any changes to a translate.csv file.
How to create an internal translation project (file upload)
How to create a managed translation project (file upload)