diff --git a/ApiCore-Backend/src/apiaries/apiaries.service.ts b/ApiCore-Backend/src/apiaries/apiaries.service.ts
index cdd4300..f838a0c 100644
--- a/ApiCore-Backend/src/apiaries/apiaries.service.ts
+++ b/ApiCore-Backend/src/apiaries/apiaries.service.ts
@@ -86,7 +86,7 @@ export class ApiariesService {
     });
     if (activeColonyCount > 0) {
       throw new ConflictException(
-        `Apiary ${id} still has ${activeColonyCount} active colonies - move or dissolve them first`,
+        `Der Bienenstand hat noch ${activeColonyCount} aktive(s) Volk/Völker - zuerst verlegen oder auflösen`,
       );
     }
     const apiary = await this.prisma.apiary.update({
diff --git a/ApiCore-Backend/src/colonies/colonies.service.ts b/ApiCore-Backend/src/colonies/colonies.service.ts
index a70c579..dfbd0ae 100644
--- a/ApiCore-Backend/src/colonies/colonies.service.ts
+++ b/ApiCore-Backend/src/colonies/colonies.service.ts
@@ -46,7 +46,7 @@ export class ColoniesService {
   private assertActive(colony: Colony): void {
     if (colony.status !== 'ACTIVE') {
       throw new ConflictException(
-        `Colony ${colony.id} is not active (status: ${colony.status})`,
+        `Das Volk ist nicht aktiv (Status: ${colony.status})`,
       );
     }
   }
@@ -61,11 +61,11 @@ export class ColoniesService {
       where: { id: apiaryId },
     });
     if (!apiary) {
-      throw new NotFoundException(`Apiary ${apiaryId} not found`);
+      throw new NotFoundException('Bienenstand nicht gefunden');
     }
     if (apiary.status !== 'ACTIVE') {
       throw new ConflictException(
-        `Apiary ${apiaryId} is not active (status: ${apiary.status})`,
+        'Der Bienenstand ist nicht aktiv und kann daher keine Völker aufnehmen',
       );
     }
   }
@@ -190,7 +190,9 @@ export class ColoniesService {
           where: { id: mergedColonyId },
         });
         if (!merged) {
-          throw new NotFoundException(`Colony ${mergedColonyId} not found`);
+          throw new NotFoundException(
+            'Zu vereinigendes Volk wurde nicht gefunden',
+          );
         }
         this.assertActive(merged);
         // currentQueenId is unique across colonies - if it stayed set on
diff --git a/ApiCore-Backend/src/queens/queens.service.ts b/ApiCore-Backend/src/queens/queens.service.ts
index fe811d1..10ec9bb 100644
--- a/ApiCore-Backend/src/queens/queens.service.ts
+++ b/ApiCore-Backend/src/queens/queens.service.ts
@@ -157,7 +157,7 @@ export class QueensService {
     const queen = await this.findById(id);
     if (QUEEN_NO_LONGER_AVAILABLE_STATUSES.includes(queen.status)) {
       throw new ConflictException(
-        `Queen ${id} is not available for assignment (status: ${queen.status}) - correct her status first if this was a mistake`,
+        'Die Königin ist nicht mehr verfügbar für eine Zuweisung - korrigiere zuerst ihren Status, falls dies ein Versehen war',
       );
     }
     const colony = await this.prisma.colony.findUnique({
diff --git a/ApiCore-Backend/src/storage/storage.service.ts b/ApiCore-Backend/src/storage/storage.service.ts
index 839fef5..2f3f812 100644
--- a/ApiCore-Backend/src/storage/storage.service.ts
+++ b/ApiCore-Backend/src/storage/storage.service.ts
@@ -134,7 +134,7 @@ export class StorageService {
     // that is the only way to correct a stock count downward.
     if (dto.type !== 'ADJUSTMENT' && dto.quantity <= 0) {
       throw new BadRequestException(
-        'quantity must be positive for IN/OUT movements - the direction is determined by "type"',
+        'Menge muss bei Eingang/Ausgang positiv sein - die Richtung wird durch den Bewegungstyp bestimmt',
       );
     }
     const delta =
diff --git a/ApiCore-Frontend/src/app/(app)/apiaries/[id]/apiary-edit-card.tsx b/ApiCore-Frontend/src/app/(app)/apiaries/[id]/apiary-edit-card.tsx
index 77e21cf..c8aa620 100644
--- a/ApiCore-Frontend/src/app/(app)/apiaries/[id]/apiary-edit-card.tsx
+++ b/ApiCore-Frontend/src/app/(app)/apiaries/[id]/apiary-edit-card.tsx
@@ -5,6 +5,7 @@ import { MapPin } from "lucide-react";
 import { useMutation, useQueryClient } from "@tanstack/react-query";
 import { toast } from "sonner";
 import { deactivateApiary, reactivateApiary, updateApiary } from "@/lib/apiaries/api";
+import { ApiError } from "@/lib/auth/api-client";
 import { Apiary } from "@/lib/apiaries/types";
 import { Button } from "@/components/ui/button";
 import { Badge } from "@/components/ui/badge";
@@ -54,7 +55,10 @@ export function ApiaryEditCard({ apiary }: { apiary: Apiary }) {
         description: "Er kann jederzeit über „Reaktivieren“ wiederhergestellt werden.",
       });
     },
-    onError: () => toast.error("Bienenstand konnte nicht deaktiviert werden"),
+    onError: (error) =>
+      toast.error(
+        error instanceof ApiError ? error.message : "Bienenstand konnte nicht deaktiviert werden",
+      ),
   });
 
   const reactivateMutation = useMutation({
diff --git a/ApiCore-Frontend/src/app/(app)/colonies/[id]/colony-actions.tsx b/ApiCore-Frontend/src/app/(app)/colonies/[id]/colony-actions.tsx
index dcfda3a..44811ac 100644
--- a/ApiCore-Frontend/src/app/(app)/colonies/[id]/colony-actions.tsx
+++ b/ApiCore-Frontend/src/app/(app)/colonies/[id]/colony-actions.tsx
@@ -3,6 +3,7 @@
 import { FormEvent, useState } from "react";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import { toast } from "sonner";
+import { ApiError } from "@/lib/auth/api-client";
 import {
   dissolveColony,
   mergeColony,
@@ -35,6 +36,10 @@ import {
   SelectValue,
 } from "@/components/ui/select";
 
+function errorMessage(error: unknown, fallback: string): string {
+  return error instanceof ApiError ? error.message : fallback;
+}
+
 function invalidateColony(
   queryClient: ReturnType<typeof useQueryClient>,
   colonyId: string,
@@ -59,6 +64,8 @@ function MoveDialog({ colony }: { colony: Colony }) {
       invalidateColony(queryClient, colony.id);
       setOpen(false);
     },
+    onError: (error) =>
+      toast.error(errorMessage(error, "Volk konnte nicht umgesetzt werden")),
   });
 
   return (
@@ -122,6 +129,8 @@ function SplitDialog({ colony }: { colony: Colony }) {
       void queryClient.invalidateQueries({ queryKey: ["colonies"] });
       setOpen(false);
     },
+    onError: (error) =>
+      toast.error(errorMessage(error, "Ableger konnte nicht angelegt werden")),
   });
 
   return (
@@ -196,6 +205,8 @@ function AssignQueenDialog({ colony }: { colony: Colony }) {
       void queryClient.invalidateQueries({ queryKey: ["queens"] });
       setOpen(false);
     },
+    onError: (error) =>
+      toast.error(errorMessage(error, "Königin konnte nicht zugewiesen werden")),
   });
 
   return (
@@ -259,6 +270,8 @@ function MergeDialog({ colony }: { colony: Colony }) {
       void queryClient.invalidateQueries({ queryKey: ["colonies"] });
       setOpen(false);
     },
+    onError: (error) =>
+      toast.error(errorMessage(error, "Völker konnten nicht vereinigt werden")),
   });
 
   return (
diff --git a/ApiCore-Frontend/src/app/(app)/queens/[id]/assign-colony-dialog.tsx b/ApiCore-Frontend/src/app/(app)/queens/[id]/assign-colony-dialog.tsx
index c652cc5..a4c9b22 100644
--- a/ApiCore-Frontend/src/app/(app)/queens/[id]/assign-colony-dialog.tsx
+++ b/ApiCore-Frontend/src/app/(app)/queens/[id]/assign-colony-dialog.tsx
@@ -4,6 +4,7 @@ import { FormEvent, useState } from "react";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import { toast } from "sonner";
 import { assignQueenColony } from "@/lib/queens/api";
+import { ApiError } from "@/lib/auth/api-client";
 import { QUEEN_UNAVAILABLE_STATUSES } from "@/lib/queens/labels";
 import { QueenStatus } from "@/lib/queens/types";
 import { fetchColonies } from "@/lib/colonies/api";
@@ -45,7 +46,10 @@ export function AssignColonyDialog({
       void queryClient.invalidateQueries({ queryKey: ["queens", queenId, "history"] });
       setOpen(false);
     },
-    onError: () => toast.error("Volk konnte nicht zugewiesen werden"),
+    onError: (error) =>
+      toast.error(
+        error instanceof ApiError ? error.message : "Volk konnte nicht zugewiesen werden",
+      ),
   });
 
   return (
diff --git a/ApiCore-Frontend/src/app/(app)/storage/record-movement-dialog.tsx b/ApiCore-Frontend/src/app/(app)/storage/record-movement-dialog.tsx
index 9e4783d..f98739b 100644
--- a/ApiCore-Frontend/src/app/(app)/storage/record-movement-dialog.tsx
+++ b/ApiCore-Frontend/src/app/(app)/storage/record-movement-dialog.tsx
@@ -3,6 +3,7 @@
 import { FormEvent, useState } from "react";
 import { useMutation, useQueryClient } from "@tanstack/react-query";
 import { addStockMovement } from "@/lib/storage/api";
+import { ApiError } from "@/lib/auth/api-client";
 import { StockMovementType } from "@/lib/storage/types";
 import { STOCK_MOVEMENT_TYPES, STOCK_MOVEMENT_TYPE_LABELS } from "@/lib/storage/labels";
 import { Button } from "@/components/ui/button";
@@ -40,8 +41,10 @@ export function RecordMovementDialog({ stockItemId }: { stockItemId: string }) {
       setQuantity("");
       setError(null);
     },
-    onError: () => {
-      setError("Bewegung konnte nicht gespeichert werden.");
+    onError: (error) => {
+      setError(
+        error instanceof ApiError ? error.message : "Bewegung konnte nicht gespeichert werden.",
+      );
     },
   });