Shaykh Ibn ‘Uthaymeen [1421H] on Speaking Slang Arabic (‘Aammiyyah)
In the Name of Allaah…
Many non-Arabs who learn proper Arabic find difficulty in communicating with regular, everyday people. The problem is that common Arabs don’t usually speak a very high level of fus-haa (proper) Arabic, rather they speak ‘Aammiyyah, a lesser grade of Arabic slang. Students of the Arabic Language tend to have a strong stance against the use of this slang, ‘Aammiyyah.
Perhaps the following words from the great scholar,Shaykh Muhammad ibn Saalih al-’Uthaymeen (may Allaah have Mercy on him), may provide some much needed insight into the issue: 
بعض الناس تجده يسرع في الكلام ويغمغم في الكلام حتى إن الإنسان يحتاج إلى أن يقول: ماذا قال؟ هذا خلاف السنة، فالسنة أن يكون الكلام بينًا واضحًا يفهمه المخاطب، وليس من الواجب أن يكون باللغة الفصحى، بل ولا من المستحب إذا كان الناس ينتقدون ذلك ويرون أن ذلك تنطع. إنما تخاطب الناس بلسانهم، وليكن كلامك بينًا واضحًا كما في حديث أنس بن مالك _ رضي الله عنه _ أن النيي _ صلى الله عليه وسلم _ كان إذا تكلم بالكلمة أعادها ثلاثًا حتى تفهم عنه
…You find that some people speak too quickly, or they mumble in their speech, and people have to ask, “What did he say?” This is against the Sunnah. The Sunnah is to make one’s speech clear and audible so the listener can understand it. It is not even a must that proper (fus-haa) Arabic is used, nor is it even recommended,  when the people are critical of this and consider it to be a kind of unnecessarily added difficulty (in communicating). Instead, speak to the people with their language, using clear, audible speech, as found in the hadeeth of Anas ibn Maalik (may Allaah be pleased with him): The Prophet (may Allaah raise his rank and grant him peace) used to repeat his words three times when speaking, so that he would be understood…
And Allaah knows best. 
 Source: His explanation of Riyaadh as-Saaliheen(4/65).
 Read the sentence carefully to avoid misunderstanding the shaykh’s point. He was not advising against the use of proper Fus-haa Arabic. Rather, he was stressing the importance of communicating with the people with words they understand.
 The hadeeth was collected by al-Bukhaaree in hisSaheeh (#94).
 Edited by Nadeem Ahsan-Shah (may Allaah bless him).
http://www.bakkah.net/en/shaykh-ibn-uth ... miyyah.htm