Complex sentence: The Thai Highlands are a mountainous natural region in the north of Thailand which is part of the range of hills extending through Laos, Burma and China and linking to the Himalayas, of which they may be considered foothills.
Split into simpler sentences: The Thai Highlands are a mountainous natural region in the north of Thailand. Its mountain ranges are part of the system of hills extending through Laos, Burma and China and linking to the Himalayas, of which they may be considered foothills.