"We want to thank our newest colleagues."
Translation:Chceme poděkovat svým nejnovějším kolegům.
I suppose it is because our is present in the original so by default it is required in the translation as well.
But I see your point "Poděkoval bratrovi." would be "Ha thanked his brother.". Svému is not strictly necessary and if it is someone else's brother, it will be clear from the context.
> and if it is someone else's brother, it will be clear from the context
As I said, I had been explained in another thread that it can be interpreted only as "svému". And if it is someone else's brother, it should be specified explicitly. Here is the thread: https://www.duolingo.com/comment/25644798
So I do not think that is should be strictly required. Like for "the" article - it is not always reqired to be present in the form of "ten/ta/to/...".