Resharper (latest version) DOES NOT underline candidate strings for localization in XAML/WPF. I could swear I saw it once but must be mistaken. I cannot find - and I have looked plenty - for a definite statement "Yes we do underline them" or "No we don't underline them". Yes - for SL no xaml localization works - got that.
The best I can see is that in the options "Element is localizable" is there for C# but not for XAML.
Would have saved me time if it was explicitly stated that resharper DOES NOT do this. The implication is they are found and underlined (your descriptions place c# and xaml on equal footing) and with so many options - we waste time looking for the magic combination.
Hope I'm wrong.