The Princess Spy by Melanie Dickerson
Margaretha has always been a romantic, and she hopes her newest suitor, Lord Claybrook, is destined to be her one true love. But then an injured man is brought to Hagenheim Castle, claiming to be an English lord who was attacked by Claybrook and left for dead. And only Mararetha---one of the few who speaks his language---understands the wild story.
Unable to pass his message on to her father, the duke, Margaretha convinces herself "Lord Colin" is just an addled stranger. But when Colin asks her to spy on Claybrook as repayment for retrieving a lost heirloom, Margaretha discovers that she might be very wrong about both Colin and her potential betrothed.
Hagenheim Series, Book #5