In the menu select "Translate" -> "Set Base Language..." and enter language identifier (ISO 639-1 Code).
In the menu select "File" -> "Open..." and select the folder with Xcode project
If you have projects with already translated strings or you received new translations, add them to the app database and then use Auto Translation feature to integrate them into your project. Add those strings by selecting in the menu "Database" -> "Scan..." and select a folder with string files [it will scan the folder and its subfolders for ".strings" files - might take some time :)]
Auto translate will try to match strings in your project with strings in the app database and if it finds matches will add them to your project, this way you can re-use already translated strings from your other projects
If you want to get a quote or translate strings with missing translations, in the menu select "Translate" -> "Export for Translation..." and save the file. It will include all strings with missing translations (even if its missing just one language).