"What a lovely day"
To say 'what a lovely day' in Italian you could use the phrase: che bella giornata. In this case, the word bella, (or beautiful) is used as a direct equivalent to describe the positive attribute of the noun.
"You look lovely in that shirt"
The equivalent of 'you look lovely in that shirt' would be stai benissimo con quella camicia, which is actually the literal translation of 'you are great with that shirt'.
The Italian word for 'love' is amore, and although there are no derivatives of the word that accurately replace the word 'lovely' in English, the use of the word amabile (or loveable) is probably as close as you are going to get.
When translating from English to Italian, its often necessary to alter the exact wording of something in order to maintain semantic integrity. An example of this would be in the vast number of American and British movies that are released in Italy under a slightly different name, with one example that pertains to our question being Amabili Resti or literally 'Loveable Remains' which was the Italian title of the 2010 film adaptation of the Alice Sebold novel 'The Lovely Bones'.
