Some insights on the typographic features

Principia has a Latin characterset which will cover most European languages. It also has some support for combining accents; so if the combination you need is not in the fonts, you might still be able to assemble it. The fonts have some nice features described here.

Standard ligatures: The lowercase f will generally try to avoid collisions and change to a shorter, shyer version of itself when there are ascenders or accents nearby. This shy f connects nicely with the next glyph. The only real ligature is an f-eth pair for Icelandic: this may not be the world’s most pressing typographic problem, but the shape is pretty.

Stylistic set: Dutch IJ: This feature changes uppercase IJ and lowercase ij pairs into a curvy, single shape inspired by examples of Dutch lettering. A proper Dutch IJ with acutes is achieved by typing Iacute + J + combining acute or I + combining acute + J + combining acute

Numerals: The default figures in LTR Principia are oldstyle, proportional and kerned. Principia has the tabular width and lining variants as well.

Case sensitive shapes: Hyphen, endash and emdash, as well as a couple of friends like parentheses, braces and brackets (and some punctuation) shift up a bit to fit nicely next to uppercase.