"I have been speaking Esperanto since the year two thousand."
Translation:Mi parolas Esperanton ekde la jaro du mil.
That would be "I had been speaking Esperanto since the year two thousand (until the period in time that we happen to be talking about now".
You're still speaking Esperanto, right? So the present tense is correct. English uses the present perfect, Esperanto (like e.g. German) the simple present.
In my experience, Esperanto uses cardinal numbers for years, like e.g. English and German, not ordinal numbers, like e.g. Russian.
So they would say, for example, that "Zamenhof was born in 1859" or "Zamenhof was born in the year 1859", not "Zamenhof was born in the 1859th year".
Parolanta is an adjective. You wouldn't say Mi parolanta any more than you would say "Mi blua."
In addition, it carries more of an active sense -- as in, you're in the state of actually speaking - so "Mi estas parolanta Esperanton" means that you're in the state of actively speaking Esperanto -- not just that speaking Esperanto is something that you do, even on a regular basis.
So "Mi parolas Esperanton ekde 2000" is the right way to say that.