It is obligatory for one to have the intention (niyyah) for the obligatory fasts prior to thestart of dawn (sunrise). However, the place of the intention is the heart, hence it is not a condition – albeit it is encouraged – for one to verbalise the intention (in whichever language you choose). If it crosses a person’s mind that he is going to fast tomorrow, in any form or manner, then s/he has made the intention. By waking up for sahur, for example, one has indirectly made the intention to fast.
Some scholars are of the view that it is sufficient to form the intention once during the beginning of Ramadan. Hence it is encouraged for all Muslims at the beginning of Ramadan to have the intention to fast everyday in Ramadan. This intention will suffice in the event that one forgets to make the niyyah completely at any day throughout Ramadan.