Are they the same fach but in a different language? Because I don't see any evidence of leggero fach in english. Could you give me some examples of Leggero singer and roles?
I think you're not seeing it around just because it's not that common a label for a singer to apply to herself. But think of it as a hybrid of a lyric and a lyric coloratura.
Specifically: the warmth of a lyric, plus the lightness and flexibility of a coloratura, with a range not necessarily as high as a coloratura.
So a good example of a leggero role would be a lyric role that doesn't push into the stratosphere but still needs plenty of agility and 'sweet' warm character, such as Gounod's Juliette.
A coloratura soprano is a type of operatic Coloratura Light soprano voice who specializes in music that is distinguished by agile runs, leaps and trills.
Lyric coloratura soprano
A very agile light voice with a high upper extension, capable of fast vocal coloratura.
The thing is that there are more than two types of soprano. The "lightest" kind of soprano, a coloratura has different subtypes, including lyric and dramatic. These are high and flexible, with a slight difference in timbre. They can sing notes really lightly and quickly, basically. Examples of coloratura sopranos include the Queen of the Night from The Magic Flute, Thais from Thais, and Norma from...Norma. Lyric sopranos, on the other hand, are more legato, warm and rich. These include the Contessa from the Marriage of Figaro and Mimi from La boheme. Of course, there are also other types. This page, which is where I got all of this info from, is very useful.