The Carroll County Board of Commissioners voted Thursday to make English the official language, capping an impassioned, four-month debate about whether the measure’s primary effect would be to save money or send a rude message to immigrants.

The unanimous vote by the five-member, all-Republican board makes Carroll the third Maryland county, after Frederick and Queen Anne’s, to pass such a law.

Commissioner Haven N. Shoemaker Jr., who introduced the ordinance Sept. 28, said his aim was to make sure the county would not be obligated to pay to translate documents into foreign languages at taxpayer expense. He said that at least 30 states have declared English their official language.

“This is normal everywhere, I guess, except for Maryland, and you can draw your own conclusions as to why that’s the case,” Shoemaker said Thursday.

Shoemaker also said the county’s public school system was paying interpreters to attend parent-teacher meetings. Carey Gaddis, a spokeswoman for the Carroll County school system, said after the meeting that interpreters are considered an essential service and receive $27.30 an hour.

CASA of Maryland and the local chapter of the NAACP joined opponents in saying the measure sent a negative message not just to immigrants but also to businesses.

Amy McNichols, a professor of Spanish at McDaniel College in Westminster, the county seat, said she was disappointed but not surprised by the board’s decision. She told the board at Thursday’s meeting and a public hearing in December that the measure was addressing a nonexistent problem, especially in light of studies that show that every wave of immigrants has overwhelmingly embraced English over time.

“What they seek to address is not really going to be a reality in the future,” she said. “And I have a lot of students who really wish they could speak their family’s language.”

The proposal touched off a fierce debate in a county whose farms and rural vistas have been changing into suburban shopping plazas and bedroom communities. The county, about 35 miles north of Baltimore, is overwhelmingly white and conservative, with 65 percent of the vote for president going to Republican Mitt Romney in November.

In a population of about 167,000, only about 4,400 residents are Latinos, the county’s fastest-growing immigrant group, according to the Census Bureau.

Few people appeared Thursday for the final vote. But more than 100 people jammed a standing-room-only hearing Dec. 11 to air their views. By unofficial count, 31 spoke against the measure; 18 argued for it.

“When someone lands from Mars, are they going to get a special sign?” demanded one speaker, who said the measure would save the county money.

Mike Stewart, 46, a lawyer who lives in Manchester, compared the ordinance to a vaccination against polio or smallpox. He said that if the county did not pass such a measure, it might find itself paying to translate government documents into a dizzying number of languages.

Stewart applauded the county for being “proactive”and said the county was wise to follow Frederick County’s example rather than the District of Columbia’s. The District, he said, spends a fortune translating documents into dozens of languages.

“Just because we don’t have a problem now, doesn’t mean we need to wait around until we have a problem,” Stewart said. He said he preferred spending money on English as a second language programs than on document translations.

But Ellen Kobler, 46, of Hampstead recalled her own decision to move to Carroll County 20 years ago and warned that the ordinance would keep more than immigrants away.

“If we were in that same position today, looking for a place to settle, and I saw a news headline or a Channel 13 story about your efforts tonight, I will tell you with certainty, we would not move to this county,” Kobler said. “The ordinance you’re proposing is not needed, and, frankly, it’s not nice.”