Comment 5 for bug 560795

However, since locl is a "smart font" OpenType feature, and Pango's raison d'ĂȘtre is to implement those features (while freetype alone only provides basic TrueType decoding), it's not really surprising that it fails with the freetype backend.

You do need pango (or more precisely harfbuzz, which is used by pango) to handle this level of font smartness.