XLIFF has been widely used for years and has proper specification. It's a (de-facto) industry standard. There are lots of tools around already to enable translators with a non-technical background to work with XLIFF.
If you want to work with professional translators, it's best for everyone to use a file format they're comfortable with. For you as a programmer it's way easier to convert stuff to a format you need, than for a translator to work with software they don't know.
I'm pretty sure that there will be some kind of compilation/conversion available in angular2 to make your translation files as small as possible.
Also once XML (which XLIFF is) is gzipped it's only about 10% bigger than an equivalent json file.