The Loco importer supports iOS strings, XLIFF files and property lists.
Localizable.strings
Loco treats files with the extension .strings as iOS strings resources and will import translations as simple key/value pairs accordingly.
XLIFF
Loco supports .xliff files exported from Xcode's "Export for Localization..." feature. XLIFF files may contain English plus a target language for translation.
Property lists
Apple property lists can be imported. Both the XML and Binary property list formats are supported as generic key/value pairs for adding translations to your project.