It is nothing to die; it is horrible not to live.

[Ce n’est rien de mourir; c’est affreux de ne pas vivre.]

Victor Hugo (1802-1885) French writer

Les Misérables, Part 5 “Jean Valjean,” Book 9 “Supreme Shadow, Supreme Dawn,” ch. 5 (5.9.5) [Jean Valjean] (1862) [tr. Wilbour/Fahnestock/MacAfee (1987)]
    (Source)

Spoken to Cosette and Marius (and his doctor) as he is dying.

(Source (French)). Alternate translations:

It is nothing to die; it is frightful not to live.
[tr. Wilbour (1862)]

It is nothing to die, but it is frightful not to live.
[tr. Wraxall (1862)]

It is nothing to die; it is dreadful not to live.
[tr. Hapgood (1887)]

To die is nothing; but it is terrible not to live.
[tr. Denny (1976)]

It’s nothing to die. It’s dreadful not to live.
[tr. Donougher (2013)]