According to the OALD:
mercurial
1 (literary) often changing or reacting in a way that is unexpected syn volatile: Emily's mercurial temperament made her difficult to live with. * Advertising is a mercurial business.

According to the GDLC:
voluble
1 Que gira fàcilment.

2 esp fig Mudable, inconstant. La voluble fortuna. Una persona, un caràcter, voluble.

The OALD suggests that 'mercurial' is a rather literary word. I'm not sure if this is the case with 'voluble' in Catalan. Perhaps we could also add some more everyday English words as equivalents as I'm not sure that every English-speaking user of the dictionary could be expected to be familiar with the English word 'mercurial' (I have to admit that 'mercurial' is in my passive vocabulary but not my active vocabulary). Perhaps we could add 'changeable', 'variable', 'fickle', even 'erratic'? Any preferences anyone?

Perhaps we could also add 'talkative' and 'garrulous' here as English equivalents? Even possibly 'loquacious' provided we mark it as 'formal'? We could maybe add an example to show that we can also use the noun 'chatterbox' here <catexamp>Es molt loquaç</catexamp><engexamp>He's very talkative / he's a real chatterbox</engexamp>

For Catalan "loquaç" I prefer talkative and loquacious (marked as formal) plus Linda's suggestion of adding the noun, chatterbox.

For me, mercurial, voluble and garrulous are all part of my passive vocab.