The English horn is a transposing instrument and sounds a fifth lower than written. Notation is in treble clef.
In modern works notation is occasionally written without transposition (sound is as written, e.g., in Hans Werner Henze).
In historical parts there are various types of notation:
Sounding as written, in alto clef (e.g., in Bach);
In bass clef, sounding an octave higher than written (in Italy from the late 18th to the middle of the 19th century, e.g., in Rossini’s William Tell Overture, so that the part could be read by a bassoonist, who played it. The bassoonist could finger the written notes as if he were playing the bassoon and the right pitches would sound on the English horn);
Sounding as written, in mezzo-soprano clef (in France between 1820 and 1840). With this notation the musician simply needs to replace the mezzo-soprano clef with the treble clef and finger the resulting pitches as usual. This automatically corresponds to modern notation, transposing by a fifth.