Language pack management

The watch stores up to four language packs. One of them, or built-in English, provides the UI translation; every installed pack contributes font glyphs (see Installed language packs). Watches advertise everything on this page with protocol capability bit 28 (multiple_language_packs_support). Older watches, and phone apps that don’t know the bit, keep using the single-pack flow. All multi-byte fields are little-endian.

Storage

Pack files use the existing pack format. Slot zero is the legacy lang file; slots 1 to 3 are lang1, lang2 and lang3 (fw/resource/language_pack.h). A slot is occupied by any file, including one whose metadata can’t be read.

Installing

Packs are installed with an ordinary PutBytes file transfer:

lang-add stores the pack in the first empty slot. If every slot is occupied, the transfer is rejected at initialization and no pack is replaced. Installing the same locale again takes another slot, so remove the old pack before installing an update. The active language doesn’t change.
lang (legacy) replaces slot zero and, once the transfer completes, selects it.

System messages

Pack management uses the System Message endpoint (0x12, fw/kernel/system_message.c). Every message starts with a zero byte and a message type, followed by a one-byte token the watch echoes in its response. Request and response structs are in fw/kernel/language_pack_protocol.h.

Message	Type	Direction
List packs	`0x0d`	phone → watch
List packs response	`0x0e`	watch → phone
Remove pack	`0x0f`	phone → watch
Remove pack response	`0x10`	watch → phone
Select language	`0x11`	phone → watch
Select language response	`0x13`	watch → phone

0x12 is not used, to avoid confusion with the endpoint ID.

List packs

The request carries only the token. The response holds the token, the slot of the active translation (0xff for English) and four 40-byte records, one per slot:

Field	Size	Description
slot	1	Slot index
flags	1	bit 0 present, bit 1 readable, bit 2 translation
version	2	Pack version, translation packs only
locale	6	ISO locale, zero-padded, translation packs only
name	30	Native language name, zero-padded, translation only

A pack is readable if it carries a translation or at least one font.

Remove pack and select language

Both requests hold the token, a slot, and the expected version and locale of the pack in that slot. Packs without a translation use zero version and locale; so does built-in English, which is selected with slot 0xff. The expected metadata stops a stale list from acting on a different pack: mismatching requests fail.

Selecting requires a pack with a translation and keeps every installed pack. Removing the selected pack selects English, even while English is toggled on. Removing a pack that a file transfer is still writing fails.

Both responses hold the token, the slot and a success byte. Requests with too-short payloads or invalid slots are answered with a failure. Additional trailing fields are ignored for compatibility with future phone apps.

PebbleOS