JAKARTA, Indonesia  Paulina Sugiarto’s three children played together at a mall here the other day, chattering not in Indonesia’s national language, but English. Their fluency often draws admiring questions from other Indonesian parents Ms. Sugiarto encounters in this city’s upscale malls.

But the children’s ability in English obscured the fact that, though born and raised in Indonesia, they were struggling with the Indonesian language, known as Bahasa Indonesia. Their parents, who grew up speaking the Indonesian language but went to college in the United States and Australia, talk to their children in English. And the children attend a private school where English is the main language of instruction.

“They know they’re Indonesian,” Ms. Sugiarto, 34, said. “They love Indonesia. They just can’t speak Bahasa Indonesia. It’s tragic.”

Indonesia’s linguistic legacy is increasingly under threat as growing numbers of wealthy and upper-middle-class families shun public schools where Indonesian remains the main language but English is often taught poorly. They are turning, instead, to private schools that focus on English and devote little time, if any, to Indonesian.