No ,is not necessary .. Fortunately, Greece is a country with a very long tradition in tourism and Rhodes its one of the most cosmopolitan island in the Aegean.
Therefore, all the people who are working in the tourism industry have a good knowledge of the English language. But even the local people living on the island have a basic understanding of the English language and are usually happy to give you any kind of information you wish. Especially among the younger, you will find most of the people speaking Italian,German in addition to English and Greek, as the island gets crowded with visitors from Italy,Germany during the high season.
So speaking Greek is not necessary...however, visitors who attempt to speak Greek are thoroughly encouraged by locals. Even if it's just a phrase or a few words, the gesture is always appreciated. Indeed, Greek is a language of gestures and to speak it also entails a lot of gesticulation. One point of confusion however, is the slight jerk of the head or eyebrows upward to indicate 'no'.
So, here are some tips in Greek that you might find of use during your stay:
Don't confuse 'yes' and 'no'. Yes is ne - which sounds like 'no' or 'nah' to English speakers. No is okhi - which sounds like 'okay' to English speakers.
Need something? Remember to say 'Please!' - parakalo. And always say 'Thank you!' - efkharisto.A general affirmative is entaksi right, okay, uh-huh.
Greece greets differently depending on the time of day. In the morning, say kalimera and in the afternoon, say kalomesimeri In the evening, say kalispera To say good night, say kalinikta.
You can also just say 'Hello' anytime - yia sas or yiassou or giasou this can also be said in parting, or as a toast. Learn some basic expressions in Greek by clicking Here