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You posted on the english tech support forum rather than the French one... as such

Your post was translated as
Hello! j 'would like to know how to change the weight d' a portions if only ' is not 1 g, 100g or 1000g? How do I if for example I eat 1 portion of dried fruit of 50g? Thank you. (English)

So.. to log a 50 gram serving of dried fruit you log 50 "servings" of 1 gram if it's available as a serving size.. or if the serving size listed is 100 grams you had 0.5 servings (1/2 in decimal form) ... 0.05 if the serving is 1000g and you had 50 grams.

and the probably butchered translation of my post into french gives

pour vous connecter à une portion de 50 grammes de fruits secs, vous vous connectez "servings 50" un gramme si it est disponible comme une portion... ou si la taille de la portion affichée est 100 grammes vous aviez 0,5 portions (1/2 sous forme décimale) (French)