diff --git a/src/components/medications/medication-card.tsx b/src/components/medications/medication-card.tsx
index f7abbabc0..1ddcd4c1a 100644
--- a/src/components/medications/medication-card.tsx
+++ b/src/components/medications/medication-card.tsx
@@ -193,7 +193,7 @@ function isLastIntakeInCurrentWindow(
 
 export function MedicationCard({ medication, onEdit }: MedicationCardProps) {
   const queryClient = useQueryClient();
-  const { t } = useTranslations();
+  const { t, locale } = useTranslations();
   const fmt = useFormatters();
   const [intakeLoading, setIntakeLoading] = useState<string | null>(null);
 
@@ -438,6 +438,7 @@ export function MedicationCard({ medication, onEdit }: MedicationCardProps) {
               {formatTimeWindowRange(
                 currentWindowStatus.schedule!.windowStart,
                 currentWindowStatus.schedule!.windowEnd,
+                locale,
               )}
             </span>
           </p>
@@ -490,7 +491,7 @@ export function MedicationCard({ medication, onEdit }: MedicationCardProps) {
                   {t("medications.nextIntake")}
                 </span>{" "}
                 {dayLabel && `${dayLabel}, `}
-                {formatTimeWindowRange(s.windowStart, s.windowEnd)}
+                {formatTimeWindowRange(s.windowStart, s.windowEnd, locale)}
                 {s.label && (
                   <span className="hidden sm:inline"> ({s.label})</span>
                 )}
diff --git a/src/components/medications/medication-form.tsx b/src/components/medications/medication-form.tsx
index 91417ed8b..32b31300c 100644
--- a/src/components/medications/medication-form.tsx
+++ b/src/components/medications/medication-form.tsx
@@ -131,16 +131,17 @@ function formatNextWindowSummary(
   schedule: Schedule,
   t: TranslateFn,
   formatShortDate: (date: Date) => string,
+  locale: "de" | "en",
 ): string {
   const isSimpleDaily =
     schedule.daysOfWeek.length === 0 && schedule.intervalWeeks === 1;
   if (isSimpleDaily) {
-    return `${t("medications.nextSchedulePrefix")} ${t("medications.nextScheduleDaily")}, ${formatTimeWindowRange(schedule.windowStart, schedule.windowEnd)}`;
+    return `${t("medications.nextSchedulePrefix")} ${t("medications.nextScheduleDaily")}, ${formatTimeWindowRange(schedule.windowStart, schedule.windowEnd, locale)}`;
   }
 
   const nextDate = getNextRecurrenceDate(schedule);
   const nextText = nextDate
-    ? `${formatShortDate(nextDate)}, ${formatTimeWindowRange(schedule.windowStart, schedule.windowEnd)}`
+    ? `${formatShortDate(nextDate)}, ${formatTimeWindowRange(schedule.windowStart, schedule.windowEnd, locale)}`
     : "—";
   return `${t("medications.nextSchedulePrefix")} ${nextText}`;
 }
@@ -234,7 +235,7 @@ export function MedicationForm({
   initial,
 }: MedicationFormProps) {
   const queryClient = useQueryClient();
-  const { t } = useTranslations();
+  const { t, locale } = useTranslations();
   const fmt = useFormatters();
 
   const doseUnits = [
@@ -565,7 +566,7 @@ export function MedicationForm({
                     `${t("medications.formSchedule")} ${i + 1}`}
                 </span>{" "}
                 <span className="text-foreground/70 font-normal">
-                  ({formatNextWindowSummary(s, t, fmt.dateShort)})
+                  ({formatNextWindowSummary(s, t, fmt.dateShort, locale)})
                 </span>
               </p>
               <DropdownMenu>
diff --git a/src/lib/__tests__/time-window-format.test.ts b/src/lib/__tests__/time-window-format.test.ts
new file mode 100644
index 000000000..106f0b28a
--- /dev/null
+++ b/src/lib/__tests__/time-window-format.test.ts
@@ -0,0 +1,44 @@
+import { describe, expect, test } from "vitest";
+
+import {
+  formatTimeWindowPart,
+  formatTimeWindowRange,
+} from "@/lib/time-window-format";
+
+describe("formatTimeWindowPart", () => {
+  test("zero-pads single-digit hours", () => {
+    expect(formatTimeWindowPart("8:00")).toBe("08:00");
+  });
+
+  test("preserves two-digit hours", () => {
+    expect(formatTimeWindowPart("19:30")).toBe("19:30");
+  });
+
+  test("returns input unchanged when not a HH:MM string", () => {
+    expect(formatTimeWindowPart("invalid")).toBe("invalid");
+  });
+});
+
+describe("formatTimeWindowRange", () => {
+  test("German locale renders 'bis ... Uhr' (default behaviour preserved)", () => {
+    expect(formatTimeWindowRange("19:00", "23:00", "de")).toBe(
+      "19:00 bis 23:00 Uhr",
+    );
+  });
+
+  test("English locale renders 'from ... to ...' instead of mixed-language 'bis Uhr'", () => {
+    // Regression for v1.4.19 F-01: app rendered "Today, 19:00 bis 23:00 Uhr"
+    // for English users because the formatter hard-coded German.
+    expect(formatTimeWindowRange("19:00", "23:00", "en")).toBe(
+      "19:00 – 23:00",
+    );
+  });
+
+  test("falls back to the German format when no locale is supplied", () => {
+    // Backwards-compat: existing call sites that have not been migrated
+    // continue to render the historic German string.
+    expect(formatTimeWindowRange("08:00", "12:00")).toBe(
+      "08:00 bis 12:00 Uhr",
+    );
+  });
+});
diff --git a/src/lib/time-window-format.ts b/src/lib/time-window-format.ts
index 037672021..858e47f82 100644
--- a/src/lib/time-window-format.ts
+++ b/src/lib/time-window-format.ts
@@ -6,6 +6,26 @@ export function formatTimeWindowPart(value: string): string {
   return `${hh}:${mm}`;
 }
 
-export function formatTimeWindowRange(start: string, end: string): string {
-  return `${formatTimeWindowPart(start)} bis ${formatTimeWindowPart(end)} Uhr`;
+/**
+ * Render a daily intake time window like "08:00 bis 12:00 Uhr" / "08:00 – 12:00".
+ *
+ * The historic call sites (see `medication-card.tsx`, `medication-form.tsx`)
+ * never passed a locale, so the renderer hard-coded the German "bis ... Uhr"
+ * suffix and the English locale ended up with mixed-language strings such as
+ * `"Today, 19:00 bis 23:00 Uhr"`. Pass the active UI locale so that the right
+ * separator + suffix are picked. When no locale is provided we keep the
+ * legacy German output for backwards compatibility with any consumer we may
+ * have missed.
+ */
+export function formatTimeWindowRange(
+  start: string,
+  end: string,
+  locale?: "de" | "en",
+): string {
+  const s = formatTimeWindowPart(start);
+  const e = formatTimeWindowPart(end);
+  if (locale === "en") {
+    return `${s} – ${e}`;
+  }
+  return `${s} bis ${e} Uhr`;
 }