Yes it was weird. However being happy for the release of your son by calling God “most gracious, the most merciful” in a language the the captors would recognize... (since it’s such a famous Islamic phrase) doesn’t seem entirely beyond reason.
Why praise God in the language of the captors? Praise God in your own language. If you must use the language of the captors then leave it at “thank you”.
BTW - Bergdahl wasn’t captured, he sought them out after first mailing home his possessions then leaving under cover of the night.
It is beyond reason. He had already been released so there's no reason to kiss up to his supposed captors.