Learn Croatian: Oni stalno pričaju o meni. - They constantly talk about me.

In Croatian, you must always use the full (emphatic) forms of pronouns after prepositions. On top of that, the preposition o requires the Locative case, and the Locative case actually only has full forms. Because of this, meni is your only option here.

Why does the preposition o take the Locative case in this sentence?

Whenever the preposition o means "about" a topic—such as talking, thinking, reading, or writing about someone or something—it always triggers the Locative case.

Could I use the verb govore instead of pričaju to mean "they talk"?

You could, but pričaju (from pričati) sounds much more natural for this context. Pričati is used for casual chatting, gossiping, or telling a story. Govoriti is typically used for the physical act of speaking, making a formal speech, or speaking a language (like "I speak English").

What is the exact meaning of stalno?

Stalno is an adverb meaning "constantly," "continually," or "all the time." It comes from the adjective stalan (constant or permanent). In Croatian, adverbs of time and frequency usually sit right before the verb, just like in this sentence.