The conjunction particle that's inserted between authors' names is determined by the bibliography style you've selected. It would appear that you're using the apalike bibliography style. Here's what you could do to change "and" to "i":
Find the file apalike.bst in your TeX distribution, make a copy, and name the copy, say, apalike-catalan.bst. (Do not edit an original file directly.)
Open apalike-catalan.bst in your favorite text editor -- the editor you use for your TeX files will do fine.
Locate the two instances of " and " in the file. (In my copy, they occur on lines 225 and 851.)
Change the " and " strings to " i " (note the spaces on either side of the conjunction).
Save the new style file, either in the same directory as where your .tex file is located or in a directory that's searched by your TeX distribution. If you choose the latter method, you will probably also need to update the filename database as is appropriate for your TeX distribution.
Be sure to include the directive \bibliographystyle{apalike-catalan} from now on. Before using the new style file, it's a good idea to delete the old aux files and then run latex, bibtex, and latex twice more to fully update all references and citations.
Happy BibTeXing!
Addendum, prompted by the OP's follow-up question: The apacite bibliography style is indeed of a much more recent vintage than apalike. If you're going to use the apacite bibliography style, you needn't (and shouldn't) modify the bibliography style file apacite.bst. Instead, you should modify some of the definitions -- specifically, \BBAA, \BBAB, and \BAnd -- set up in the file apacite.sty. The following MWE (minimum working example) illustrates what you should do when using apacite to get the "correct" conjunction particles and the intended language is Catalan:
\documentclass{article}
\usepackage{apacite}
\bibliographystyle{apacite}
\renewcommand{\BBAA}{i} % between authors in parenthetical cites and ref. list
\renewcommand{\BBAB}{i} % between authors in in-text citation
\renewcommand{\BAnd}{i} % for ``Ed. \& Trans.'' in ref. list
\begin{document}
\cite{whatever:2002} % provide correct citation key(s)
\bibliography{mybibfile} % provide name of actual .bib file (or files)
\end{document}
Finally, note that the apacite package provides sets of language-specific adjustments -- look for files with extension .apc -- for Dutch, English, Finnish, French, German (and ngerman), Greek, Norwegian, Spanish, and Swedish. I guess it's fair to assume that the author(s) of the apacite package would warmly welcome a Catalan addition. :-)
