"unflagging devotion" is an appropriate expression in written English.
You can use it to describe someone's dedication to an important cause or pursuit. For example, you might say, "Her unflagging devotion to the cause kept her going, even in the face of challenging odds."
In a statement from the White House, Barack Obama said the king was a tireless champion of his country's development, who "demonstrated unflagging devotion to improving the standard of living of the Thai people".
He is a man driven by passions large and small that include a deep love of Israel ("Sidney has never gotten around to worrying about Gentiles," says a longtime friend) and unflagging devotion to the New York Giants.
Dr. Martin's unflagging devotion to the cancer patient is why he was universally revered by colleagues and trainees alike.
Her unflagging optimism, her devotion to Horace Mann and love of her family are an inspiration to all.
Popper married Josephine Anna Henninger ('Hennie') in 1930, and she oversaw his welfare with unflagging support and devotion, serving additionally as his amanuensis until her death in 1985.
Back in his home village, Lin Kong has a wife whose bound feet are the sign of her backwardness, but whose devotion is unflagging, while in Muji he keeps company with an educated nurse named Manna Wu.
Ms. Cargill's sprightly beauty and Mr. Buechele's unflagging pursuit helped throughout their courtship, but so did the religious devotion they shared.
The officers, board and staff of the American Technion Society ATSS) mourn the loss of Alex J. Etkin of Detroit, whose six decades of devotion to our organization stand as a beacon of unflagging support.
His determination to achieve his brothers' goals was genuine and unrelenting, his devotion to each of his own many causes over the years was sincere and unflagging.
William Dazeley is an unflagging, rather charmless Don.
But market pressures are real and unflagging.
