I have created the German language file

[Mati1199]

{
"extensionName": { "message": "Anwendungsmenü" },
"description": { "message": "Eine WebExtension, die das Anwendungsmenü der Firefox-Titelleiste hinzufügt." },

"browserAction":	{ "message":	"Firefox"	},

"popup_newTab":	{ "message":	"Neuer Tab"	},
"popup_newWindow":	{ "message":	"Neues Fenster"	},
"popup_newPrivateWindow":	{ "message":	"Neues privates Fenster"	},
"popup_openFile":	{ "message":	"Datei öffnen…"	},

"popup_edit":	{ "message":	"Bearbeiten"	},
"popup_editCut":	{ "message":	"Ausschneiden"	},
"popup_editCopy":	{ "message":	"Kopieren"	},
"popup_editPaste":	{ "message":	"Einfügen"	},
"popup_editUndo":	{ "message":	"Rückgängig machen"	},
"popup_editRedo":	{ "message":	"Wiederherstellen"	},
"popup_editSelectAll":	{ "message":	"Alles auswählen"	},
"popup_editDelete":	{ "message":	"Löschen"	},

"popup_findInPage":	{ "message":	"Seite durchsuchen…"	},
"popup_savePageAs":	{ "message":	"Seite sichern als…"	},
"popup_emailLink":	{ "message":	"Link per Email senden…"	},

"popup_print":	{ "message":	"Drucken…"	},
"popup_printPreview":	{ "message":	"Druckvorschau"	},
"popup_pageSetup":	{ "message":	"Seite einrichten…"	},

"popup_fullScreen":	{ "message":	"Vollbild"	},
"popup_sync":	{ "message":	"Bei Sync anmelden"	},
"popup_restart":	{ "message":	"Neustart"	},
"popup_exit":	{ "message":	"Beenden"	},

"popup_bookmarks":	{ "message":	"Lesezeichen"	},
"popup_history":	{ "message":	"Chronik"	},
"popup_downloads":	{ "message":	"Downloads"	},
"popup_addons":	{ "message":	"Add-ons"	},
"popup_options":	{ "message":	"Optionen"	},
"popup_help":	{ "message":	"Hilfe"	},

"popup_dev":	{ "message":	"Web-Entwickler"	},
"popup_devTools":	{ "message":	"Werkzeuge ein-/ausblenden"	},
"popup_devInspector":	{ "message":	"Inspektor"	},
"popup_devConsole":	{ "message":	"Web-Konsole"	},
"popup_devDebugger":	{ "message":	"Debugger"	},
"popup_devStyleEditor":	{ "message":	"Stilbearbeitung"	},
"popup_devPerformance":	{ "message":	"Laufzeitanalyse"	},
"popup_devNetwork":	{ "message":	"Netzwerk"	},
"popup_devStorageInspector":	{ "message":	"Speicher-Inspektor"	},
"popup_devDOM":	{ "message":	"DOM"	},
"popup_devToolbar":	{ "message":	"Entwickler-Symbolleiste"	},
"popup_devWebIDE":	{ "message":	"WebIDE"	},
"popup_devBrowserToolbox":	{ "message":	"Browser Toolbox"	},
"popup_devBrowserContentToolbox":	{ "message":	"Browser Content Toolbox"	},
"popup_devBrowserConsole":	{ "message":	"Browser-Konsole"	},
"popup_devResponsiveDesign":	{ "message":	"Bildschirmgrößen testen"	},
"popup_devEyedropper":	{ "message":	"Farbpipette"	},
"popup_devScratchpad":	{ "message":	"JavaScript-Umgebung"	},
"popup_devServiceWorkers":	{ "message":	"Service-Worker"	},
"popup_devConnect":	{ "message":	"Verbinden…"	},
"popup_devPageSource":	{ "message":	"Seitenquelltext anzeigen"	},
"popup_devGetTools":	{ "message":	"Weitere Tools laden"	},
"popup_workOffline":	{ "message":	"Offline arbeiten "	},

"popup_optionsToolbar_menu":	{ "message":	"Symbolleisten"	},
"popup_optionsToolbar_navigation":	{ "message":	"Menüleiste"	},
"popup_optionsToolbar_bookmarks":	{ "message":	"Lesezeichen-Symbolleiste"	},
"popup_optionsCustomize":	{ "message":	"Anpassen…"	},

"popup_helpFirefox":	{ "message":	"Firefox-Hilfe"	},
"popup_helpGettingStarted":	{ "message":	"Erste Schritte"	},
"popup_helpTour":	{ "message":	"Tour durch Firefox"	},
"popup_helpShortcuts":	{ "message":	"Tastenkombinationen"	},
"popup_helpHealthReport":	{ "message":	"Firefox-Statusbericht"	},
"popup_helpTroubleshootingInformation":	{ "message":	"Informationen zur Fehlerbehebung"	},
"popup_helpSubmitFeedback":	{ "message":	"Feedback senden…"	},
"popup_restartInSafeMode":	{ "message":	"Mit deaktivierten Add-ons neu starten…"	},
"popup_helpAbout":	{ "message":	"Über Firefox"	},

"options_header":	{ "message":	"Optionen"	},
"options_theme":	{ "message":	"Themes:"	},
"options_preferredWindowState":	{ "message":	"Bevorzugte Fenstergröße:"	},
"options_preferredWindowState_maximized":	{ "message":	"Maximiert"	},
"options_preferredWindowState_normal":	{ "message":	"Normales Fenster"	}

}

[Mati1199]

Sorry, my English is not good and I can not cope with the website.