The joint
English earnest and Ernest are the same sound. The plot is the pun: the women are in love with the name because the name sounds like the virtue. Translate the virtue only, and the plot loses its reason. Translate the name only, and the title is a puzzle.
German is the lucky lane. Ernst is a name and ernst means serious. The sound-play survives. What thins is the virtue: earnestness is steadfast sincerity, and ernst is seriousness. Close enough for the play to work, not a photocopy.
French usually renames him Constant, a man and a steadfastness. The machine is the same kind of machine, a name that is an adjective, and the adjective is not Wilde’s. That is a successful compensation. It is also a different title. Spanish La importancia de llamarse Ernesto keeps a name, changes “to be” into “to be called,” and lets the homophone die. The play then has to carry, in the dialogue, a joke the title no longer tells.
Why this break matters to a reader
A reader of the Spanish title is promised a story about why a name matters, and is not told that the name means the virtue the heroines are buying. A reader of the French title is promised a different virtue, constancy, which the English play was not named for. A reader of the German title is nearest the stall Wilde set up. The pun is the plot. If it breaks, the plot needs a footnote, or a new pun, to stand.