Dialectal is the adjective corresponding to the noun dialect, and dialect means means a variety of a language peculiar to a particular region or group. Dialectical corresponds to the noun dialectic, which refers to (1) the art or practice of finding truth through logical argument, and (2) the process of change through the interaction of contradictory forces. It’s often used synonymously with logic or philosophy. So the ic differentiating dialectal and dialectical makes a big difference.
Examples
Though dialectic is the dictionary-approved adjective corresponding to dialect, dialectical is often used in its place (the reverse error is rare)—for example:
Voice Actions was always uncannily good at interpreting input and transcribing messages, but … I wonder how well it’ll do at working with dialectical nuances. [Washington Post]
While most people know of [the verb "birth"] only in a dialectical sense (“Miss Scarlett, I don’t know nothing about birthing no babies!”), it was quite common in the Middle Ages. [Columbia Journalism Review]
And these writers use dialectic and dialectical correctly:
A careful observer of character vocal and physical rhythms, of dialectal and gestural markers, he gives us a picture of the real Americans he encountered … [Oregon Live]
They are kind of dialectical. They feed on each other. [transcribed in Forbes]
However, he stresses that there is a huge difference between slang and dialectal colloquialisms. [Scottish Daily Record]
. . . the pitmen prove themselves surprisingly fluent in dialectical debate. [New York Times]
