What we call "tense" is actually a pretty fuzzy category, and at a general level is just the "grammaticalization of time reference", meaning the transition of a word from the expression of concrete content to an abstract time-related function word. This in all languages is a constantly evolving process and can occur over a long period of time and to different degrees. English hasn't even fully grammaticalized the notion of the future, as our future and present tenses are morphologically identical.
Which brings up another point: both English and Mandarin lack elaborate morphological change, relying on auxiliaries to convey tense rather than explicit verbal modifications. Where do we draw the line here? At the end of the day, was the collapse of the Roman empire due to the richness of the morphological tense marking of Latin?
Also, that would be fine grammatically in English, but it would connote "I can't attend because I regularly go to a seminar, which today conflicts with the meeting, and I will go the the seminar in preference to it." If that's not what you meant, though -- say, if it's a one-time thing -- then it would be wrong in the sense of failing to communicate your intent.
There's also "I'm to go to a seminar".
That seems like a pretty poor (no pun intended) example, given that "I am going to a seminar" uses present tense (present progressive) to describe a future event.
(A different reading of the above quote suggests that the second form is supposed to be "I am going to go to a seminar.")