In Spanis, if you want to say "It is raining" you must "Esta lloviendo", you can't use the prounoun in this type of sentences. Does the same apply to Greek?
Yes, it's like there is no subject, not even vaguely 'it' like in English.
Having said that, in more poetic expressions, you may see something like 'the sky is raining (something)'.
That's interesting. βρέχω is also ancient Gk, as in Matt 5:45 καὶ βρέχει ἐπὶ δικαίους καὶ ἀδίκους, "and it rains upon the righteous and the unrighteous." The use of ὕει (impersonal) for "it rains" seems related to the verb ὑετίζω (LSJM). It occurs in Herodotus at 3.117 τὸν μὲν γὰρ χειμῶνα ὕει σφι ὁ θεὸς, "for the god sends them rain in the winter." The passage in Theognis 25 uses the participle (οὐδὲ γὰρ ὁ Ζεὺς οὔθ᾽ ὕων πάντεσσ᾽ ἁνδάνει οὔτ᾽ ἀνέχων). But Theocritus 4.43 also has the impersonal verb: χὡ Ζεὺς ἄλλοκα μὲν πέλει αἴθριος, ἄλλοκα δ᾽ ὕει, "Zeus in one place makes it clear, elswhere he brings rain."
The word "Ναι" here in Brasil (specifically in the North Region) sometimes is used in some sentences like: "Isso aqui está bom, né". The word "né" is a colloquial way to say "não é?!" but this is just a curiosity!