First start command message of a BOT doesn't receive the language of the user client
Start a new bot
Bot receive the start message update without language_code field in the user object.
Further message updates received from same user will contain the language_code value
Initial message update received by the bot contains the language_code field for the user.
Developer can use the language_code in start message to personalize initial message for users into their own language so they can correctly understand it.