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"ممكن"  is more commonly used for request in Egyptian , "من فضلك " is very polite and formal way so it's not commonly used in every day's situations; it's sth like "would you please say it again?". Also, it's used in fosha instead

Please speak slowly
  ممكن تتكلم بشويش/ ممكن تتكلم بالراحة

What is the comparison of فنله
In English?

we usually use the word "قميص" for shirt, and just "تي شيرت" for t-shirt 

but "فنلة" or "فلنة" (it can be read either ways) is used for sleeveless tops especially those people wear under their clothes or in sports

The best way to recognize such things is searching google images :) 

Questions

 من فضلك تكلم بشويش (بالراحة

What is the comparison of فنله
In English?

U can say t.shirt تيشرت

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What do you mean by saying فنله ?

Anywho, I corrected those sentences as an Egyptian, this is how we actually speak, I don't know if you want them in Fusha or not.

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Questions

Are these correct?

How do you say it in Arabic?
ازاي تقولوها بالعربي؟

How do you spell it?
ازاي تستهجها

Please say it again.
من فضلك قولها تاني

Please speak slowly
ممكن تكلمني على مهلك

 فنلة