SEOUL—The English-language version of a voice-activated “virtual assistant” that is a significant feature of Samsung Electronics Co. ’s newest flagship device won’t be ready to go when the Galaxy S8 smartphone arrives in U.S. stores next week.

The delayed English-language rollout of the Galaxy S8’s artificial-intelligence service, dubbed Bixby, could stretch on until as late as the end of May, a person familiar with the matter said. No final decision has been made on timing, people familiar with the matter said.

During recent internal tests, the performance of Bixby’s voice recognition in English has lagged behind that of the virtual assistant’s performance in Korean, the people familiar with the matter said.

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In response to an inquiry from The Wall Street Journal, a spokeswoman for Samsung said that while other Bixby features would be available when the phone is released on April 21, the voice-activated Bixby service would only become available in the U.S. “later this spring.” The spokeswoman didn’t comment on the reasons for the delay.

Bixby, akin to Apple Inc.’s Siri, is an artificial-intelligence service used to field tasks. Samsung has made the service an important selling point of the South Korean technology company’s new smartphone. It introduced a button on the side of the device devoted to Bixby, while extolling its “intelligent interface and contextual awareness.”

The Bixby delay threatens to damp some of the enthusiasm for the Galaxy S8, whose sleek design has garnered strong reviews. In the buildup to the April 21 sales-launch day, Samsung had heavily touted Bixby, whose functions include voice recognition. Bixby, for example, can complete multiple tasks with a single voice command, such as locating a nearby steakhouse and hailing a taxi.

Industry experts doubt that a postponed Bixby launch would hurt sales significantly, given that initial enthusiasm for the Galaxy S8 smartphone has focused more on its sleek aesthetics. Others said it was too early to make a prediction.

“Rushing with a half-baked solution to the market will actually discourage users to use Bixby,” said Neil Shah, research director at Counterpoint Research, which tracks smartphone shipments.

Counterpoint Research estimates Samsung will sell more than 50 million Galaxy S8 handsets—more than the S7 model, which was a best seller for the company. “I don’t think Bixby is a Holy Grail feature which will hamper Galaxy S8 sales because eventually via software updates users will receive it,” Mr. Shah said.

Samsung’s decision to delay Bixby’s voice-activation features, even by several weeks, comes after years of the smartphone maker pushing aggressively to meet promised launch dates.

Samsung’s approach cost it last year when Galaxy Note 7 handsets began having battery problems, with some catching fire. Executives rushed to recall the devices, but after reissuing replacement handsets, those, too, had battery malfunctions. The misstep cost Samsung $5 billion, rattled consumer trust and tarnished the company’s premium brand image.

Its new flagship phone shows some signs of restraint: In a break with prior launches, the Galaxy S8 doesn’t feature a more-powerful battery. After the Galaxy Note 7 recall, Samsung promised it would avoid future manufacturing blunders by creating an eight-point quality plan for smartphones, among other measures.

The Galaxy S8 is Samsung’s most closely watched product launch in years, and Bixby had been a highly promoted addition. Bixby has a variety of functions beyond its voice-activated features. It also has image recognition, so a user can take a photo of a bottle of wine and then learn what nearby stores sell it.

Samsung users don’t yet have much enthusiasm for features like Bixby, with three-quarters saying they don’t have a preference or don’t care if a phone has an intelligent assistant, according to a survey last month by Fluent LLC, a marketing technology company.

“It’s just not that big of a deal yet,” said Jordan Cohen, chief marketing officer at Fluent, speaking of features like Bixby or Siri. “Mostly, people want a phone to be waterproof or have better battery life.”

For Samsung, Bixby represents the company’s first major foray into trying to compete in the growing field of artificial intelligence. Last year, it acquired U.S. startup Viv Labs Inc., which was founded by the co-creators of Siri, to bolster its capabilities, and executives have promised that the Galaxy S8 would offer services that are “significantly differentiated” from those of the competition.

Write to Timothy W. Martin at timothy.martin@wsj.com