diff --git a/src/composables/useGetMessages.ts b/src/composables/useGetMessages.ts
index 3edb3606a23..c1f7ca91ae0 100644
--- a/src/composables/useGetMessages.ts
+++ b/src/composables/useGetMessages.ts
@@ -27,6 +27,7 @@ import { EventBus } from '../services/EventBus.ts'
 import { useChatStore } from '../stores/chat.ts'
 import { useChatExtrasStore } from '../stores/chatExtras.ts'
 import { debugTimer } from '../utils/debugTimer.ts'
+import { tryLocalizeSystemMessage } from '../utils/message.ts'
 import { useGetThreadId } from './useGetThreadId.ts'
 import { useGetToken } from './useGetToken.ts'
 
@@ -632,6 +633,19 @@ export function useGetMessagesProvider() {
 			return
 		}
 
+		if (message.systemMessage !== '' && conversation.value) {
+			// Attempt to localize non-system messages
+			try {
+				message.message = tryLocalizeSystemMessage({
+					message,
+					conversation: conversation.value,
+				})
+			} catch (exception) {
+				tryPollNewMessages()
+				return
+			}
+		}
+
 		// Patch for federated conversations: disable unsupported file shares
 		if (conversation.value?.remoteServer && Object.keys(message.messageParameters ?? {}).some((key) => key.startsWith('file'))
 			&& [MESSAGE.TYPE.COMMENT, MESSAGE.TYPE.VOICE_MESSAGE, MESSAGE.TYPE.RECORD_VIDEO, MESSAGE.TYPE.RECORD_AUDIO].includes(message.messageType)) {
diff --git a/src/constants.ts b/src/constants.ts
index 88356d2a5c3..0b9b408a892 100644
--- a/src/constants.ts
+++ b/src/constants.ts
@@ -169,6 +169,8 @@ export const ATTENDEE = {
 	BOT_PREFIX: 'bot-',
 	BRIDGE_BOT_ID: 'bridge-bot',
 
+	ACTOR_CLI_ID: 'cli',
+	ACTOR_SYSTEM_ID: 'system',
 	CHANGELOG_BOT_ID: 'changelog',
 	SAMPLE_BOT_ID: 'sample',
 } as const
diff --git a/src/utils/message.ts b/src/utils/message.ts
index 8c130d97dd7..ae730326421 100644
--- a/src/utils/message.ts
+++ b/src/utils/message.ts
@@ -3,9 +3,162 @@
  * SPDX-License-Identifier: AGPL-3.0-or-later
  */
 
-import type { ChatMessage } from '../types/index.ts'
+import type { ChatMessage, Conversation } from '../types/index.ts'
 
-import { ATTENDEE, MESSAGE } from '../constants.ts'
+import { t } from '@nextcloud/l10n'
+import { ATTENDEE, CONVERSATION, MENTION, MESSAGE } from '../constants.ts'
+
+/**
+ * Module augmentation to declare the t function from `@nextcloud/l10n`
+ * (no placeholders should be replaced until it reaches NcRichText)
+ *
+ * @param app - app name ('spreed')
+ * @param text - translated string
+ */
+declare module '@nextcloud/l10n' {
+	export function t(app: string, text: string): string
+}
+
+/**
+ * Returns correct mention type for self actor
+ */
+const SELF_MENTION_TYPE = {
+	[ATTENDEE.ACTOR_TYPE.USERS]: MENTION.TYPE.USER,
+	[ATTENDEE.ACTOR_TYPE.FEDERATED_USERS]: MENTION.TYPE.FEDERATED_USER,
+	[ATTENDEE.ACTOR_TYPE.EMAILS]: MENTION.TYPE.EMAIL,
+	[ATTENDEE.ACTOR_TYPE.GUESTS]: MENTION.TYPE.GUEST,
+} as const
+
+type SelfMentionTypeKey = keyof typeof SELF_MENTION_TYPE
+
+/**
+ * Sync with server-side constant SYSTEM_MESSAGE_TYPE_RELAY in lib/Signaling/Listener.php
+ */
+const SYSTEM_MESSAGE_TYPE_RELAY = [
+	MESSAGE.SYSTEM_TYPE.CALL_STARTED, // 'call_started',
+	MESSAGE.SYSTEM_TYPE.CALL_JOINED, // 'call_joined',
+	MESSAGE.SYSTEM_TYPE.CALL_LEFT, // 'call_left',
+	MESSAGE.SYSTEM_TYPE.CALL_ENDED, // 'call_ended',
+	MESSAGE.SYSTEM_TYPE.CALL_ENDED_EVERYONE, // 'call_ended_everyone',
+	MESSAGE.SYSTEM_TYPE.THREAD_CREATED, // 'thread_created',
+	MESSAGE.SYSTEM_TYPE.THREAD_RENAMED, // 'thread_renamed',
+	MESSAGE.SYSTEM_TYPE.MESSAGE_DELETED, // 'message_deleted',
+	MESSAGE.SYSTEM_TYPE.MESSAGE_EDITED, // 'message_edited',
+	MESSAGE.SYSTEM_TYPE.MODERATOR_PROMOTED, // 'moderator_promoted',
+	MESSAGE.SYSTEM_TYPE.MODERATOR_DEMOTED, // 'moderator_demoted',
+	MESSAGE.SYSTEM_TYPE.GUEST_MODERATOR_PROMOTED, // 'guest_moderator_promoted',
+	MESSAGE.SYSTEM_TYPE.GUEST_MODERATOR_DEMOTED, // 'guest_moderator_demoted',
+	MESSAGE.SYSTEM_TYPE.FILE_SHARED, // 'file_shared',
+	MESSAGE.SYSTEM_TYPE.OBJECT_SHARED, // 'object_shared',
+	MESSAGE.SYSTEM_TYPE.HISTORY_CLEARED, // 'history_cleared',
+	MESSAGE.SYSTEM_TYPE.POLL_VOTED, // 'poll_voted',
+	MESSAGE.SYSTEM_TYPE.POLL_CLOSED, // 'poll_closed',
+	MESSAGE.SYSTEM_TYPE.RECORDING_STARTED, // 'recording_started',
+	MESSAGE.SYSTEM_TYPE.RECORDING_STOPPED, // 'recording_stopped',
+] as const
+
+/**
+ * System messages that aren't necessary to translate
+ */
+const SYSTEM_MESSAGE_TYPE_UNTRANSLATED = [
+	MESSAGE.SYSTEM_TYPE.REACTION,
+	MESSAGE.SYSTEM_TYPE.REACTION_DELETED,
+	MESSAGE.SYSTEM_TYPE.REACTION_REVOKED,
+	MESSAGE.SYSTEM_TYPE.MESSAGE_DELETED,
+	MESSAGE.SYSTEM_TYPE.MESSAGE_EDITED,
+	MESSAGE.SYSTEM_TYPE.THREAD_CREATED,
+	MESSAGE.SYSTEM_TYPE.THREAD_RENAMED,
+] as const
+
+/**
+ * System messages that aren't shown separately in the chat
+ */
+const SYSTEM_MESSAGE_TYPE_HIDDEN = [
+	MESSAGE.SYSTEM_TYPE.REACTION,
+	MESSAGE.SYSTEM_TYPE.REACTION_DELETED,
+	MESSAGE.SYSTEM_TYPE.REACTION_REVOKED,
+	MESSAGE.SYSTEM_TYPE.POLL_VOTED,
+	MESSAGE.SYSTEM_TYPE.MESSAGE_DELETED,
+	MESSAGE.SYSTEM_TYPE.MESSAGE_EDITED,
+	MESSAGE.SYSTEM_TYPE.THREAD_CREATED,
+	MESSAGE.SYSTEM_TYPE.THREAD_RENAMED,
+] as const
+
+/**
+ * Returns whether the actor is CLI (Administrator)
+ *
+ * @param message Chat message
+ */
+function cliIsActor(message: ChatMessage) {
+	return message.messageParameters.actor.id === ATTENDEE.ACTOR_CLI_ID
+		&& message.messageParameters.actor.type === MENTION.TYPE.GUEST
+}
+
+/**
+ * Returns whether the conversation is one-to-one
+ *
+ * @param type conversation type
+ */
+function conversationIsOneToOne(type: number) {
+	return type === CONVERSATION.TYPE.ONE_TO_ONE
+		|| type === CONVERSATION.TYPE.ONE_TO_ONE_FORMER
+}
+
+/**
+ * Returns whether the call is started silently
+ * FIXME should be coming with the message metadata from HPB
+ *
+ * @param message conversation type
+ */
+function callIsSilent(message: ChatMessage) {
+	return /* $metaData[Message::METADATA_SILENT] */false
+}
+
+/**
+ * Returns whether the actor is current user (You)
+ *
+ * @param actorId user id
+ * @param actorType user type
+ */
+function selfMentionId(actorId: string, actorType: string) {
+	if (actorType === ATTENDEE.ACTOR_TYPE.GUESTS) {
+		return `${MENTION.TYPE.GUEST}/${actorId}`
+	}
+	return actorId
+}
+
+/**
+ * Returns whether the actor is current user (You)
+ *
+ * @param actorType user type
+ */
+function selfMentionType(actorType: string) {
+	return SELF_MENTION_TYPE[actorType as SelfMentionTypeKey] ?? MENTION.TYPE.GUEST
+}
+
+/**
+ * Returns whether the actor is current user (You)
+ *
+ * @param message Chat message
+ * @param selfActorId Current user id
+ * @param selfActorType Current user type
+ */
+function selfIsActor(message: ChatMessage, selfActorId: string, selfActorType: string) {
+	return message.messageParameters.actor.id === selfActorId
+		&& message.messageParameters.actor.type === selfMentionType(selfActorType)
+}
+
+/**
+ * Returns whether the actor is current user (you)
+ *
+ * @param message Chat message
+ * @param selfActorId Current user id
+ * @param selfActorType Current user type
+ */
+function selfIsUser(message: ChatMessage, selfActorId: string, selfActorType: string) {
+	return message.messageParameters.user.id === selfMentionId(selfActorId, selfActorType)
+		&& message.messageParameters.user.type === selfMentionType(selfActorType)
+}
 
 /**
  * Returns whether the given system message should be hidden in the UI
@@ -17,19 +170,144 @@ export function isHiddenSystemMessage(message: ChatMessage): boolean {
 	// System message for auto unpin
 	if (message.systemMessage === MESSAGE.SYSTEM_TYPE.MESSAGE_UNPINNED
 		&& message.actorType === ATTENDEE.ACTOR_TYPE.GUESTS
-		&& message.actorId === 'system'
+		&& message.actorId === ATTENDEE.ACTOR_SYSTEM_ID
 	) {
 		return true
 	}
 
-	return [
-		MESSAGE.SYSTEM_TYPE.REACTION,
-		MESSAGE.SYSTEM_TYPE.REACTION_DELETED,
-		MESSAGE.SYSTEM_TYPE.REACTION_REVOKED,
-		MESSAGE.SYSTEM_TYPE.POLL_VOTED,
-		MESSAGE.SYSTEM_TYPE.MESSAGE_DELETED,
-		MESSAGE.SYSTEM_TYPE.MESSAGE_EDITED,
-		MESSAGE.SYSTEM_TYPE.THREAD_CREATED,
-		MESSAGE.SYSTEM_TYPE.THREAD_RENAMED,
-	].includes(message.systemMessage)
+	return SYSTEM_MESSAGE_TYPE_HIDDEN.includes(message.systemMessage)
+}
+
+/**
+ * Returns whether the given system message should be hidden in the UI
+ *
+ * @param payload Chat message
+ * @param payload.message Chat message
+ * @param payload.conversation Current conversation
+ * @return whether the message is hidden in the UI
+ */
+export function tryLocalizeSystemMessage({
+	message,
+	conversation,
+}: { message: ChatMessage, conversation: Conversation }): string {
+	if (SYSTEM_MESSAGE_TYPE_UNTRANSLATED.includes(message.systemMessage)) {
+		// Don't localize hidden system messages, keep original
+		return message.message
+	}
+
+	if (!SYSTEM_MESSAGE_TYPE_RELAY.includes(message.systemMessage)) {
+		// Don't localize non-supported relayed system messages, do polling
+		throw new Error()
+	}
+
+	switch (message.systemMessage) {
+		case MESSAGE.SYSTEM_TYPE.CALL_STARTED: {
+			if (callIsSilent(message)) {
+				if (selfIsActor(message, conversation.actorId, conversation.actorType)) {
+					return conversationIsOneToOne(conversation.type)
+						? t('spreed', 'Outgoing silent call')
+						: t('spreed', 'You started a silent call')
+				} else {
+					return conversationIsOneToOne(conversation.type)
+						? t('spreed', 'Incoming silent call')
+						: t('spreed', '{actor} started a silent call')
+				}
+			} else {
+				if (selfIsActor(message, conversation.actorId, conversation.actorType)) {
+					return conversationIsOneToOne(conversation.type)
+						? t('spreed', 'Outgoing call')
+						: t('spreed', 'You started a call')
+				} else {
+					return conversationIsOneToOne(conversation.type)
+						? t('spreed', 'Incoming call')
+						: t('spreed', '{actor} started a call')
+				}
+			}
+		}
+		case MESSAGE.SYSTEM_TYPE.CALL_JOINED: {
+			return selfIsActor(message, conversation.actorId, conversation.actorType)
+				? t('spreed', 'You joined the call')
+				: t('spreed', '{actor} joined the call')
+		}
+		case MESSAGE.SYSTEM_TYPE.CALL_LEFT: {
+			return selfIsActor(message, conversation.actorId, conversation.actorType)
+				? t('spreed', 'You left the call')
+				: t('spreed', '{actor} left the call')
+		}
+		case MESSAGE.SYSTEM_TYPE.CALL_ENDED:
+		case MESSAGE.SYSTEM_TYPE.CALL_ENDED_EVERYONE: {
+			// TODO discuss if worth localizing at all or only basic cases
+			// #parseCall - 150 lines of PHP code
+			// requires to know amount of guests, duration, $maxDurationWasReached
+			// Alternatives: parse message.message directly for this information
+			// Simpler alternatives: just return t('spreed', message.message)
+
+			// Can be simplified to this:
+			// if ($message === 'call_ended') {
+			// $subject = $this->l->t('Call ended (Duration {duration})');
+			// } else {
+			// $subject = $this->l->t('{actor} ended the call (Duration {duration})');
+			// }
+			throw new Error()
+		}
+		case MESSAGE.SYSTEM_TYPE.MODERATOR_PROMOTED:
+		case MESSAGE.SYSTEM_TYPE.GUEST_MODERATOR_PROMOTED: {
+			if (selfIsActor(message, conversation.actorId, conversation.actorType)) {
+				return t('spreed', 'You promoted {user} to moderator')
+			} else if (selfIsUser(message, conversation.actorId, conversation.actorType)) {
+				return cliIsActor(message)
+					? t('spreed', 'An administrator promoted you to moderator')
+					: t('spreed', '{actor} promoted you to moderator')
+			}
+			return cliIsActor(message)
+				? t('spreed', 'An administrator promoted {user} to moderator')
+				: t('spreed', '{actor} promoted {user} to moderator')
+		}
+		case MESSAGE.SYSTEM_TYPE.MODERATOR_DEMOTED:
+		case MESSAGE.SYSTEM_TYPE.GUEST_MODERATOR_DEMOTED: {
+			if (selfIsActor(message, conversation.actorId, conversation.actorType)) {
+				return t('spreed', 'You demoted {user} from moderator')
+			} else if (selfIsUser(message, conversation.actorId, conversation.actorType)) {
+				return cliIsActor(message)
+					? t('spreed', 'An administrator demoted you from moderator')
+					: t('spreed', '{actor} demoted you from moderator')
+			}
+			return cliIsActor(message)
+				? t('spreed', 'An administrator demoted {user} from moderator')
+				: t('spreed', '{actor} demoted {user} from moderator')
+		}
+		case MESSAGE.SYSTEM_TYPE.FILE_SHARED:
+		case MESSAGE.SYSTEM_TYPE.OBJECT_SHARED: {
+			// Backend transforms both 'file_shared' and 'object_shared' to normal chat message,
+			// these should not be received by client
+			throw new Error()
+		}
+		case MESSAGE.SYSTEM_TYPE.HISTORY_CLEARED: {
+			return selfIsActor(message, conversation.actorId, conversation.actorType)
+				? t('spreed', 'You cleared the history of the conversation')
+				: t('spreed', '{actor} cleared the history of the conversation')
+		}
+		case MESSAGE.SYSTEM_TYPE.POLL_VOTED: {
+			return t('spreed', 'Someone voted on the poll {poll}')
+		}
+		case MESSAGE.SYSTEM_TYPE.POLL_CLOSED: {
+			return selfIsActor(message, conversation.actorId, conversation.actorType)
+				? t('spreed', 'You ended the poll {poll}')
+				: t('spreed', '{actor} ended the poll {poll}')
+		}
+		case MESSAGE.SYSTEM_TYPE.RECORDING_STARTED: {
+			return selfIsActor(message, conversation.actorId, conversation.actorType)
+				? t('spreed', 'You started the video recording')
+				: t('spreed', '{actor} started the video recording')
+		}
+		case MESSAGE.SYSTEM_TYPE.RECORDING_STOPPED: {
+			return selfIsActor(message, conversation.actorId, conversation.actorType)
+				? t('spreed', 'You stopped the video recording')
+				: t('spreed', '{actor} stopped the video recording')
+		}
+		default: {
+			// Don't localize non-supported relayed system messages, do polling
+			throw new Error()
+		}
+	}
 }