Yealink SIP-T54S Administrator's Manual page 295

The following shows a portion of the language pack "000.GUI.English.lang" for the phone
user interface (take SIP-T46G IP phones for example):
Save the language file and place it to the provisioning server (e.g., 192.168.10.25).
3.
Specify the access URL of the phone user interface language pack in the configuration
4.
files.
If you want to add a new custom language (e.g., Guilan) to your IP phone (e.g., SIP-T46G),
prepare the language file named as "011.GUI.Guilan.lang" for downloading. After update, you
will find a new language selection "Guilan" on the IP phone user interface:
Menu->Basic->Language.
Procedure
Loading language pack can only be performed using the configuration files.
Central Provisioning
(Configuration File)
<y0000000000xx>.cfg
Configuring Basic Features
Specify the access URL of the
phone user interface language
pack.
Parameter:
gui_lang.url
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