InFlight Classic 

Message: Interface\AddOns\InFlight\InFlight.lua:121: attempt to index upvalue 'vars' (a nil value)
Time: Fri Nov  1 09:08:40 2019
Count: 3
Stack: Interface\AddOns\InFlight\InFlight.lua:121: attempt to index upvalue 'vars' (a nil value)
Interface\AddOns\InFlight\InFlight.lua:121: in function <Interface\AddOns\InFlight\InFlight.lua:115>
[C]: ?

Locals: button = TaxiButton4 {
 0 = <userdata>
 inflighted = true
}
id = 4
(*temporary) = nil
(*temporary) = "Grom'gol"
(*temporary) = "REACHABLE"
(*temporary) = nil
(*temporary) = "attempt to index upvalue 'vars' (a nil value)"
TaxiNodeGetType = <function> defined =[C]:-1
vars = nil
taxiSrc = "Grom'gol"
ShortenName = <function> defined @Interface\AddOns\InFlight\InFlight.lua:38
TaxiNodeName = <function> defined =[C]:-1
addDuration = <function> defined @Interface\AddOns\InFlight\InFlight.lua:105
GetEstimatedTime = <function> defined @Interface\AddOns\InFlight\InFlight.lua:57

on InFlight.lua i added:
	if gameLocale == "deDE" then
		self:Localize()	
	end
and before this line:
	self.db = LibStub("AceDB-3.0"):New("InFlightDB", self.defaults, true)

and added the function:

---------------------------------------
function InFlight:Localize()
---------------------------------------
	local faction = UnitFactionGroup("player")
	local db = self.defaults.global[faction]
	local dbLocalized = {}
	
	for srcName, targets in pairs(db) do
		local srcNameLocalized = L[srcName]
		
		if (srcNameLocalized) then
			dbLocalized[srcNameLocalized] = {}
			
			for targetName, targetTime in pairs(targets) do
				local targetNameLocalized = L[targetName]
				
				if (targetNameLocalized) then
					dbLocalized[srcNameLocalized][targetNameLocalized] = targetTime
				end
			end
		end
	end
	
	self.defaults.global[faction] = dbLocalized
end

on de-de.lua i haved added localized flightpoints:
L["Booty Bay"] = "Beutebucht"
L["Refuge Pointe"] = "Die Zuflucht"
L["Everlook"] = "Everlook"
L["Moonglade"] = "Moonglade"
L["Nighthaven Village"] = "Moonglade"
L["Talonbranch Glade"] = "Nachtlaublichtung"
L["Thorium Point"] = "Thoriumspitze"
L["Auberdine"] = "Auberdine"
L["Aerie Peak"] = "Aerie Peak"
L["Light's Hope Chapel"] = "Kapelle des hoffnungvollen Lichts"
L["Lakeshire"] = "Seenhain"
L["Morgan's Vigil"] = "Morgans Wacht"
L["Menethil Harbor"] = "Hafen von Menethil"
L["Ironforge"] = "Ironforge"
L["Theramore"] = "Theramore"
L["Chillwind Camp"] = "Chillwind Lager"
L["Rut'theran Village"] = "Rut'theran"
L["Ratchet"] = "Ratchet"
L["Thalanaar"] = "Thalanaar"
L["Gadgetzan"] = "Gadgetzan"
L["Darkshire"] = "Dunkelhain"
L["Nethergarde Keep"] = "Feste Schhreckensfels"
L["Thelsamar"] = "Thelsamar"
L["Talrendis Point"] = "Talrendisspitze"
L["Nijel's Point"] = "Die Niejelspitze"
L["Stormwind"] = "Stormwind"
L["Stonetalon Peak"] = "Der Steinkrallengipfel"
L["Sentinel Hill"] = "Späherkuppe"
L["Astranaar"] = "Astranaar"
L["Cenarion Hold"] = "Burg Cenarius"
L["Feathermoon"] = "Feathermoon"
L["Southshore"] = "Southshore"
L["Marshal's Refuge"] = "Marshals Zuflucht"

L["Camp Mojache"] = "Camp Mojache"
L["Crossroads"] = "Crossroads"
L["Freewind Post"] = "Freiwindposten"
L["Sunrock Retreat"] = "Sonnenfels"
L["Orgrimmar"] = "Orgrimmar"
L["Shadowsprey Village"] = "Shadowprey"
L["Thunder Bluff"] = "Thunder Bluff"
L["Valormak"] = "Valormok"
L["Bloodvenom Post"] = "Blutgiftposten"
L["Splintertree Post"] = "Splintertreeposten"
L["The Zoram Strand"] = "Zoram'gar-Außenposten"
L["Grom’gol"] = "Grom'gol"
L["Hammerfall"] = "Hammerfall"
L["Kargath"] = "Kargath"
L["Stonard"] = "Stonard"
L["Tarren Mill"] = "Tarrens Mühle"
L["The Sepulcher"] = "Das Grabmal"
L["Undercity"] = "Unterstadt"
L["Lights Hope Chappel"] = "Kapelle des hoffnungvollen Lichts"
L["Revantusk Village"] = "Revantusk"
L["Flame Crest"] = "Flammenkamm"