Nope, you need to have contents in Catalan in your .cat site, not for six months but forever. I couldn't find this 6-months clause anywhere, and the terms don't mention it at all: http://fundacio.cat/ca/domini/sobre-el-cat/normativa
Furthermore, they clearly say that violation of the only usage restriction (contents in Catalan) can result in your domain being suspended.
edit: Just a little correction to your text in Catalan: L'idioma català és una llengua bonica. ;)
>> I couldn't find this 6-months clause anywhere ..
Actually if you try to add a *.cat domain through gandi.net, they included this clause as a tip-text encoded in the little warning image: "Warning: the registration contract for domain names in this extension require that you publish at least one page in Catalan within 6 months."
Furthermore, they clearly say that violation of the only usage restriction (contents in Catalan) can result in your domain being suspended.
edit: Just a little correction to your text in Catalan: L'idioma català és una llengua bonica. ;)