diff --git a/lib/Conversion/ConversionProvider.php b/lib/Conversion/ConversionProvider.php
index 75c38bb423..195e2ff38b 100644
--- a/lib/Conversion/ConversionProvider.php
+++ b/lib/Conversion/ConversionProvider.php
@@ -9,6 +9,7 @@
 
 namespace OCA\Richdocuments\Conversion;
 
+use OCA\Richdocuments\Service\LanguageService;
 use OCA\Richdocuments\Service\RemoteService;
 use OCA\Richdocuments\Service\SecureViewService;
 use OCP\Files\Conversion\ConversionMimeProvider;
@@ -56,6 +57,7 @@ public function __construct(
 		private LoggerInterface $logger,
 		IFactory $l10nFactory,
 		private SecureViewService $secureViewService,
+		private LanguageService $languageService,
 	) {
 		$this->l10n = $l10nFactory->get('richdocuments');
 	}
@@ -168,17 +170,10 @@ public function convertFile(File $file, string $targetMimeType): mixed {
 		return $this->remoteService->convertFileTo(
 			$file,
 			$targetFileExtension,
-			conversionOptions: ['lang' => $this->getConversionLanguage()]
+			conversionOptions: ['lang' => $this->languageService->getBCP47LanguageTag()]
 		);
 	}
 
-	private function getConversionLanguage(): string {
-		$locale = $this->l10n->getLocaleCode();
-		$language = $locale !== '' ? $locale : $this->l10n->getLanguageCode();
-
-		return str_replace('_', '-', $language);
-	}
-
 	private function getMimeProvidersFor(array $inputMimeTypes, string $outputMimeType): array {
 		$outputMimeInfo = $this->getMimeInfoFor($outputMimeType);
 		if ($outputMimeInfo === null) {
diff --git a/lib/Service/InitialStateService.php b/lib/Service/InitialStateService.php
index 7eef2b47ae..00826ff041 100644
--- a/lib/Service/InitialStateService.php
+++ b/lib/Service/InitialStateService.php
@@ -28,6 +28,7 @@ public function __construct(
 		private TemplateManager $templateManager,
 		private CapabilitiesService $capabilitiesService,
 		private IConfig $config,
+		private LanguageService $languageService,
 		private ?string $userId,
 	) {
 	}
@@ -42,6 +43,7 @@ public function provideCapabilities(): void {
 		$this->initialState->provideInitialState('hasNextcloudBranding', $this->capabilitiesService->hasNextcloudBranding());
 		$this->initialState->provideInitialState('instanceId', $this->config->getSystemValue('instanceid'));
 		$this->initialState->provideInitialState('wopi_callback_url', $this->appConfig->getNextcloudUrl());
+		$this->initialState->provideInitialState('bcp47Language', $this->languageService->getBCP47LanguageTag());
 		$this->provideOptions();
 
 		$this->hasProvidedCapabilities = true;
diff --git a/lib/Service/LanguageService.php b/lib/Service/LanguageService.php
new file mode 100644
index 0000000000..8b00aaad04
--- /dev/null
+++ b/lib/Service/LanguageService.php
@@ -0,0 +1,58 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OCA\Richdocuments\Service;
+
+use OCA\Richdocuments\AppInfo\Application;
+use OCP\L10N\IFactory;
+
+class LanguageService {
+	private const LOCALE_OVERRIDES = [
+		'de' => [
+			'de_CH' => 'de-CH',
+			'gsw' => 'de-CH',
+			'gsw_CH' => 'de-CH',
+		],
+		'fr' => [
+			'fr_CH' => 'fr-CH',
+		],
+		'it' => [
+			'it_CH' => 'it-CH',
+		],
+	];
+
+	public function __construct(
+		private IFactory $l10nFactory,
+	) {
+	}
+
+	/**
+	 * Converts the current user's Nextcloud language/locale settings into the
+	 * BCP 47 language tag Collabora Online expects.
+	 */
+	public function getBCP47LanguageTag(): string {
+		$l10n = $this->l10nFactory->get(Application::APPNAME);
+
+		// getLanguageCode()/getLocaleCode() mirror @nextcloud/l10n's getLanguage()/getLocale()
+		$language = str_replace('_', '-', $l10n->getLanguageCode());
+		$locale = $l10n->getLocaleCode();
+
+		$language = match ($language) {
+			'de-DE' => 'de', // German formal should just be treated as 'de'
+			'es-419' => 'es-MX', // not a valid locale string in COOL
+			default => $language,
+		};
+
+		if ($language === 'en-GB' && $locale === 'en_AU') {
+			$language = 'en-AU';
+		}
+
+		return self::LOCALE_OVERRIDES[$language][$locale] ?? $language;
+	}
+}
diff --git a/src/components/CoolFrame.vue b/src/components/CoolFrame.vue
index 579a153030..81b0f49c23 100644
--- a/src/components/CoolFrame.vue
+++ b/src/components/CoolFrame.vue
@@ -28,7 +28,7 @@
 <script>
 
 import { generateCSSVarTokens, getCollaboraTheme, getUITheme } from '../helpers/coolParameters.js'
-import { languageToBCP47 } from '../helpers/index.js'
+import { loadState } from '@nextcloud/initial-state'
 import PostMessageService from '../services/postMessage.tsx'
 
 export default {
@@ -74,7 +74,7 @@ export default {
 		window.addEventListener('message', this.handlePostMessage)
 
 		if (this.iframeUrl.length > 0) {
-			this.formAction = this.iframeUrl + '?lang=' + languageToBCP47()
+			this.formAction = this.iframeUrl + '?lang=' + loadState('richdocuments', 'bcp47Language', '')
 			this.isIframeLoaded = true
 		} else {
 			return
diff --git a/src/helpers/index.js b/src/helpers/index.js
index 7545a9133f..4933c0d41c 100644
--- a/src/helpers/index.js
+++ b/src/helpers/index.js
@@ -3,52 +3,6 @@
  * SPDX-License-Identifier: AGPL-3.0-or-later
  */
 
-import { getLanguage, getLocale } from '@nextcloud/l10n'
-
-const languageToBCP47 = () => {
-	let language = getLanguage().replace(/_/g, '-')
-	const locale = getLocale()
-
-	// German formal should just be treated as 'de'
-	if (language === 'de-DE') {
-		language = 'de'
-	}
-
-	// es-419 should be mapped as this is not considered a valid locale string in COOL
-	if (language === 'es-419') {
-		language = 'es-MX'
-	}
-
-	// Australia
-	if (language === 'en-GB' && locale === 'en_AU') {
-		language = 'en-AU'
-	}
-
-	// special case where setting the bc47 region depending on the locale setting makes sense
-	const whitelist = {
-		de: {
-			de_CH: 'de-CH',
-			gsw: 'de-CH',
-			gsw_CH: 'de-CH',
-		},
-		fr: {
-			fr_CH: 'fr-CH',
-		},
-		it: {
-			it_CH: 'it-CH',
-		},
-	}
-	const matchingWhitelist = whitelist[language]
-	if (typeof matchingWhitelist !== 'undefined' && typeof matchingWhitelist[locale] !== 'undefined') {
-		return matchingWhitelist[locale]
-	}
-
-	// Collabora Online expects BCP47 language tag syntax.
-	// When the Nextcloud language consists of two parts, we send both,
-	// as the region is then provided by the language setting.
-	return language
-}
-
 const getNextcloudVersion = () => {
 	return parseInt(OC.config.version.split('.')[0])
 }
@@ -68,7 +22,6 @@ const getRandomId = (length = 5) => {
 }
 
 export {
-	languageToBCP47,
 	getNextcloudVersion,
 	splitPath,
 	getRandomId,
diff --git a/src/helpers/url.js b/src/helpers/url.js
index 85090bbf7a..c6d66d591d 100644
--- a/src/helpers/url.js
+++ b/src/helpers/url.js
@@ -5,7 +5,6 @@
 
 import { getRootUrl, generateUrl } from '@nextcloud/router'
 import { getSharingToken } from '@nextcloud/sharing/public'
-import { languageToBCP47 } from './index.js'
 import Config from './../services/config.tsx'
 
 const getSearchParam = (name) => {
@@ -39,7 +38,7 @@ const getWopiUrl = ({ fileId, readOnly, closeButton, revisionHistory, target = u
 	//   https://<loolwsd-server>:9980/hosting/discovery
 	return Config.get('urlsrc')
 		+ 'WOPISrc=' + encodeURIComponent(getWopiSrc(fileId))
-		+ '&lang=' + languageToBCP47()
+		+ '&lang=' + Config.get('bcp47Language')
 		+ (closeButton ? '&closebutton=1' : '')
 		+ (revisionHistory ? '&revisionhistory=1' : '')
 		+ (readOnly ? '&permission=readonly' : '')
diff --git a/src/services/config.tsx b/src/services/config.tsx
index 759b69e051..eb1cf46a19 100644
--- a/src/services/config.tsx
+++ b/src/services/config.tsx
@@ -11,6 +11,7 @@ class ConfigService {
 	constructor() {
 		this.values = {
 			wopi_callback_url: loadState('richdocuments', 'wopi_callback_url', ''),
+			bcp47Language: loadState('richdocuments', 'bcp47Language', ''),
 			...loadState('richdocuments', 'document', {}),
 		}
 	}
diff --git a/tests/lib/Conversion/ConversionProviderTest.php b/tests/lib/Conversion/ConversionProviderTest.php
index ddf87b29d8..6f498f7165 100644
--- a/tests/lib/Conversion/ConversionProviderTest.php
+++ b/tests/lib/Conversion/ConversionProviderTest.php
@@ -10,6 +10,7 @@
 namespace Tests\Richdocuments\Conversion;
 
 use OCA\Richdocuments\Conversion\ConversionProvider;
+use OCA\Richdocuments\Service\LanguageService;
 use OCA\Richdocuments\Service\RemoteOptionsService;
 use OCA\Richdocuments\Service\RemoteService;
 use OCA\Richdocuments\Service\SecureViewService;
@@ -26,6 +27,7 @@ class ConversionProviderTest extends TestCase {
 	private IFactory&MockObject $l10nFactory;
 	private IL10N&MockObject $l10n;
 	private SecureViewService&MockObject $secureViewService;
+	private LanguageService&MockObject $languageService;
 	private ConversionProvider $provider;
 
 	protected function setUp(): void {
@@ -36,6 +38,7 @@ protected function setUp(): void {
 		$this->l10nFactory = $this->createMock(IFactory::class);
 		$this->l10n = $this->createMock(IL10N::class);
 		$this->secureViewService = $this->createMock(SecureViewService::class);
+		$this->languageService = $this->createMock(LanguageService::class);
 
 		$this->l10n->method('t')->willReturnCallback(static fn (string $text): string => $text);
 		$this->l10nFactory->method('get')
@@ -47,20 +50,21 @@ protected function setUp(): void {
 			$this->logger,
 			$this->l10nFactory,
 			$this->secureViewService,
+			$this->languageService,
 		);
 	}
 
 	public function testConvertFilePassesCurrentLocaleToCollabora(): void {
 		$file = $this->createMock(File::class);
 
-		$this->l10n->expects($this->once())
-			->method('getLocaleCode')
-			->willReturn('de_DE');
+		$this->languageService->expects($this->once())
+			->method('getBCP47LanguageTag')
+			->willReturn('de');
 		$this->secureViewService->method('isEnabled')
 			->willReturn(false);
 		$this->remoteService->expects($this->once())
 			->method('convertFileTo')
-			->with($file, 'pdf', RemoteOptionsService::REMOTE_TIMEOUT_DEFAULT, ['lang' => 'de-DE'])
+			->with($file, 'pdf', RemoteOptionsService::REMOTE_TIMEOUT_DEFAULT, ['lang' => 'de'])
 			->willReturn('pdf-content');
 
 		$result = $this->provider->convertFile($file, 'application/pdf');
diff --git a/tests/lib/Service/LanguageServiceTest.php b/tests/lib/Service/LanguageServiceTest.php
new file mode 100644
index 0000000000..e11a32f789
--- /dev/null
+++ b/tests/lib/Service/LanguageServiceTest.php
@@ -0,0 +1,61 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace Tests\Richdocuments\Service;
+
+use OCA\Richdocuments\AppInfo\Application;
+use OCA\Richdocuments\Service\LanguageService;
+use OCP\IL10N;
+use OCP\L10N\IFactory;
+use PHPUnit\Framework\MockObject\MockObject;
+use PHPUnit\Framework\TestCase;
+
+class LanguageServiceTest extends TestCase {
+	private IFactory&MockObject $l10nFactory;
+	private IL10N&MockObject $l10n;
+	private LanguageService $service;
+
+	protected function setUp(): void {
+		parent::setUp();
+
+		$this->l10nFactory = $this->createMock(IFactory::class);
+		$this->l10n = $this->createMock(IL10N::class);
+		$this->l10nFactory->method('get')
+			->with(Application::APPNAME)
+			->willReturn($this->l10n);
+
+		$this->service = new LanguageService($this->l10nFactory);
+	}
+
+	public static function languageProvider(): array {
+		return [
+			'plain pass-through' => ['en', 'en_US', 'en'],
+			'German formal is treated as plain German' => ['de_DE', 'de_DE', 'de'],
+			'es-419 is mapped to es-MX for COOL' => ['es_419', 'es_419', 'es-MX'],
+			'Australian English' => ['en_GB', 'en_AU', 'en-AU'],
+			'British English without Australian locale stays en-GB' => ['en_GB', 'en_GB', 'en-GB'],
+			'Swiss German locale' => ['de', 'de_CH', 'de-CH'],
+			'Swiss German (gsw locale)' => ['de', 'gsw', 'de-CH'],
+			'Swiss German (gsw_CH locale)' => ['de', 'gsw_CH', 'de-CH'],
+			'Swiss French locale' => ['fr', 'fr_CH', 'fr-CH'],
+			'Swiss Italian locale' => ['it', 'it_CH', 'it-CH'],
+			'German without Swiss locale stays de' => ['de', 'de_DE', 'de'],
+		];
+	}
+
+	/**
+	 * @dataProvider languageProvider
+	 */
+	public function testGetBCP47LanguageTag(string $languageCode, string $localeCode, string $expected): void {
+		$this->l10n->method('getLanguageCode')->willReturn($languageCode);
+		$this->l10n->method('getLocaleCode')->willReturn($localeCode);
+
+		$this->assertSame($expected, $this->service->getBCP47LanguageTag());
+	}
+}