To learn more about the localization take a look at these articles:
What is Localization
How to Localize
To localize strings in RadDataGrid you need to use the GridLocalizationManager class.
Key
Description
And
Relates to the Filtering UI.
CaseSensitiveMode
ClearFilter
Contains
DoesNotContain
DragToGroup
EndsWith
EqualsTo
Filter
IsFalse
IsGreaterThan
IsGreaterThanOrEqualTo
IsLessThan
IsLessThanOrEqualTo
IsTrue
Or
StartsWith