feat: support more langs

* add translations for French, Spanish, and Italian
* change the wording for better understanding of this feature:
    - "import" to "restore"
    - "export" to "backup"
* rename functions to reflect these changes

src/www/js/i18n.js

@@ -11,8 +11,6 @@ const messages = { // eslint-disable-line no-unused-vars
     clients: 'Clients',
     new: 'New',
     deleteClient: 'Delete Client',
-    upload: 'Upload',
-    download: 'Download',
     deleteDialog1: 'Are you sure you want to delete',
     deleteDialog2: 'This action cannot be undone.',
     cancel: 'Cancel',
@@ -32,6 +30,10 @@ const messages = { // eslint-disable-line no-unused-vars
     donate: 'Donate',
     toggleCharts: 'Show/hide Charts',
     theme: { dark: 'Dark theme', light: 'Light theme', auto: 'Auto theme' },
+    restore: 'Restore',
+    backup: 'Backup',
+    titleRestoreConfig: 'Restore your configuration',
+    titleBackupConfig: 'Backup your configuration',
   },
   ua: {
     name: 'Ім`я',
@@ -195,6 +197,10 @@ const messages = { // eslint-disable-line no-unused-vars
     downloadConfig: 'Télécharger la configuration',
     madeBy: 'Développé par',
     donate: 'Soutenir',
+    restore: 'Restaurer',
+    backup: 'Sauvegarder',
+    titleRestoreConfig: 'Restaurer votre configuration',
+    titleBackupConfig: 'Sauvegarder votre configuration',
   },
   de: { // github.com/florian-asche
     name: 'Name',
@@ -279,6 +285,10 @@ const messages = { // eslint-disable-line no-unused-vars
     donate: 'Donar',
     toggleCharts: 'Mostrar/Ocultar gráficos',
     theme: { dark: 'Modo oscuro', light: 'Modo claro', auto: 'Modo automático' },
+    restore: 'Restaurar',
+    backup: 'Realizar copia de seguridad',
+    titleRestoreConfig: 'Restaurar su configuración',
+    titleBackupConfig: 'Realizar copia de seguridad de su configuración',
   },
   ko: {
     name: '이름',
@@ -495,6 +505,10 @@ const messages = { // eslint-disable-line no-unused-vars
     downloadConfig: 'Scarica configurazione',
     madeBy: 'Realizzato da',
     donate: 'Donazione',
+    restore: 'Ripristina',
+    backup: 'Backup',
+    titleRestoreConfig: 'Ripristina la tua configurazione',
+    titleBackupConfig: 'Esegui il backup della tua configurazione',
   },
   th: {
     name: 'ชื่อ',