Alternate translations:
mouche de renfort aux fentes latérales
or
renfort sur la couture latérale du bas

Explantion:
I was not able to find an explanation of the gusset at the hem of a shirt on the Internet so I went to a book in my textile library, Shirtmaking: Developing Skills for Fine Sewing by David Page Coffin. You might be able to see the page of the book on line at Amazon.com. Select "look inside this book" and do a search for gusset; then select page 123, which has the illustration and explanation.

The hem gusset is a small, triangular or diamond shaped patch of material that reinforces the side seam at the hem.

Fortunately, I was able to find a photograph of the "hirondelle de renfort" and I can confirm that it is the reinforcing gusset which is discussed in Coffin's book.

References:
Shirtmaking: Developing Skills for Fine Sewing
by David Page Coffin
illustration and explanation of reinforcing gusset - page 123
photograph of reinforcing gusset - page 127
http://www.amazon.com/gp/reader/1561582646/ref=sib_dp_ptu#

Timberland Long Sleeve Cotton/Linen Striped Woven
# 60% linen/40% conventional cotton stripe plain weave.
# Garment washed with enzymes.
# All over pre-wrinkled effect.
# Signature gusset at the hem.
http://www.6pm.com/n/p/p/7307773/c/6497.html

Hirondelle de renfort [see picture]
http://chatolufsen.blogspot.com/

A noter également les hirondelles de renfort assortis au bas de la chemise.
http://www.chemise-homme.com/ficheproduit-chemise-homme-cein...

Hirondelles de renfort.
http://www.chemise-homme.com/ficheproduit-chemise-homme-cein...

RUSSEL 130g/m², 100%coton sergé, col traditionnel, col de propreté en mailles chevrons ton sur ton, poches de poitrine plaquées boutonnées avec rabat, manches courtes avec patte boutonnée, renfort ton sur ton sur la couture latérale du bas, ourlet droit sur le devant et arrondi au dos
http://www.design-broderie.com/tarif-chemise.htm

Très belle chemise homme italienne, coupe classique légèrement ajustée, aux finitions raffinées :
- col classique italien
- pin point
- bas de chemise classique (arrondi)
- boutons nacre véritable
- "mouche" de renfort aux fentes latérales
http://www.private-avenue.com/chemise-homme-italienne-uni-bl...


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Note added at 1 day18 hrs (2009-03-18 14:56:50 GMT) Post-grading
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You're welcome, Alejandra2007, and thank you! A la prochaine!