The diet for people with diabetes is the same healthy diet recommended for everyone so you should be able to choose items from the usual menu whilst away from home. If you generally follow a healthy diet there is no harm in making some higher fat, higher sugar choices whilst on holiday. If you do want something that isn't on the menu, for example jacket or boiled potato instead of chips or vegetables served without butter, ask the staff. Even if they can't provide it there and then, most are willing to oblige their guests if they can.
It isn't necessary but if there are certain foods you feel that you want to make sure your holiday destination provide, for example skimmed milk or diet coke, you should let them know in advance. If you are traveling alone, you may like to let the staff know when you check-in as a precaution in case you are taken unwell during your stay.
Food abroad, in any country, contains the same basic ingredients - starchy foods, fruit and vegetables, protein and dairy foods. You should be able to choose from the local menu and still eat a balanced diet. Holidays are also an ideal time to experiment with different foods and there is not reason why you should not fully enjoy the wide variety of food available.
If you are flying to your holiday destination remember that there is no need to order a special 'diabetic meal' on board. They are often low in carbohydrate so are generally unsuitable. On a longer haul flight it is worth taking some extra snacks with you - airline meals tend to be smaller than the kind of meals we all normally eat.