I know Splunk has support for these languages for the REST API.
Do all the APIs having the same capabilities?
Say, the most basic function that is to login, which is very generic.
So far I have seen Java and C# functionalities, but they don't seems to mirror-image one another.
Is this true?
The SDKs aren't necessarily identical in their features. Underneath they all use Splunk's REST API which can be called directly from any one of those languages if your SDK is missing a feature you need.