Answer No. 1
Language variety and change actually the term that used in sociolinguistics that refer the small variations which occur in language and which are determined by external, social factors. These variations can and also do lead in time to change of language. They contrast with variations in language which are motivated by internal factors structural features of a language which can also lead to change, especially when this internal variation occurs during first language acquisition. According to Holmes (1992), the cause behind language change is the variation of use in the areas of pronunciation and vocabulary.
There are two results of linguistic change that investigate by Holmes (1992) which involves the spread of a sound which has prestige in New York, but interestingly has no prestige in London, as follows:
1.      Post-vocal | r | its spread and its status
In many parts of England and Wales, Standard English has lost the pronunciation post-vocal  r . The loss of  r  began in the 17 th  century in the South-East of England and is still spreading to other areas. Accents with post-vocal | r | are called  rhotict , and these accents are regarded as rural and uneducated. In large parts of the USA, on the other hand, post-vocalic | r | is alive and well and extensively used. On the other hand in cities like New York, pronouncing the letter  r  is regarded as prestigious.
2.      The spread of vernacular forms
Sometimes a vernacular feature in some communities as a reflection of ethnic or social identity such as what happened in Martha's Vineyard Island. Labov's 1960 study showed; when the island was invaded by summer tourists, the island community of fishermen changed their pronunciation of some word vowels to older forms from the past as a reaction to the language of tourists.
Holmes in his book, said that there are three reasons how do the changes of language spread, as follows:
From group to group.  The changes spread like waves in different directions, and social factors such as age, gender, status and social group affect the rates and directions of change.
From style to style.  When a change is a prestigious one, it usually starts at the top of the speech community - in the most formal style of the highest status group and spreads downwards. Style which from more formal to more casual, from one individual to another, from one social group to another, and from one word to another.
Lexical diffusion.  Lexical Diffusion refers to sound changes typically spread through different words one by one.The change from one word's vowel to another, the sound change begins in one word and later on in another, etc.
According to Holmes (1992), there are four reasons for the language change, namely:
Social status and language change.  The members of the group with most social status could be the first reason for the language change, for example, tend to introduce changes into a speech community from neighboring communities which have greater status and prestige in their eyes.
Gender and change . The differences in women's and men's speech are a source of variation which can result in linguistic change.
Interaction and language change . Interaction and contact between people is crucial in providing the channels for linguistic change (social networks).
The influence of the media . New forms can gain prestige from the media. TV may have played a part in explaining at least the speed with which pronunciations. Some researcher belief that media has a great influence on people's speech patterns and new forms.
Answer No. 2
A language is dead when it no longer has any speakers. Language death is defined by Campbell as  "the loss of a language due togradual shift to the dominant language in language contact situations"  (1994:1961). When we talk about what actually that causing the death, loss and decay of languange, a long list of such factors can be found in Campbell (1994:1963). The most commonly cited are socioeconomic and sociopolitical. Meanwhile in Wurm (1991), there are 5