THE Coca-Cola Company yesterday launched the Hong Kong stage of an advertising campaign it bravely hopes will help to displace tea as the most popular drink in Chinese restaurants. The campaign is spearheaded by 30, 20-second to 30-second television commercials that cost US$20 million to produce. The advertisements were made mostly in the United States by Creative Artists Agency of California and eight were adapted for Hong Kong by McCann Erickson. Swire Bottlers' general manager Paul McDonald said: ''We don't measure ourselves against Pepsi. In Hong Kong, our major competitor is tea, or tap water.'' He said Coca-Cola was the biggest single unit of product sales in supermarkets in Hong Kong. It was a bigger seller than rice. But there was still room for expanding sales: in Chinese restaurants ''people still reach for tea''. The television adswill begin airing in Hong Kong from next Monday.