Add "Elevation gain / loss" es-ES and fr-FR translations

Based on help from computers & friends, I'm sure both know better than I
do!

src/locales/es-ES.json

@@ -11,7 +11,7 @@
   "Show all": "Mostrar todos",
   "Select device": "Seleccionar dispositivo",
   "Distance travelled": "Distancia recorrida",
-  "Elevation gain / loss": "",
+  "Elevation gain / loss": "Aumento / disminución de la altura",
   "Download raw data": "Descargar datos en crudo",
   "Information": "Información",
   "Show last known locations": "Mostrar última ubicación conocida",

src/locales/fr-FR.json

@@ -11,7 +11,7 @@
   "Show all": "Tout afficher",
   "Select device": "Sélectionner un appareil",
   "Distance travelled": "Distance parcourue",
-  "Elevation gain / loss": "",
+  "Elevation gain / loss": "Augmentation / diminution de l'altitude",
   "Download raw data": "Télécharger les données brutes",
   "Information": "Informations",
   "Show last known locations": "Afficher les dernières localisations connues",