Not sure where you can get a definition of language, but I suppose it could be described as something that can be taught (as in a subject for GCSE or the like, not as you might get taught at acting school for a part in a play)whereas a dialect is something more specific to a particular area, an example is the use of the term 'mardy baby' which is common from nottingham upwards, but is - or used to be- rarely understood further south, in fact this is a term I was told about years ago, and because it is so descriptive I have used it even though I'm a Londoner, but someone I met who also knew the term immediately thought I was from Nottingham. The various terms used for others, such as love, me duck etc. are also I think dialect type variations, some of which cause offence when applied to people of another area!