Hindi: kachrah

This is a word used in Bahrain and Kuwait and probably other Gulf countries to describe something of poor quality. Usually for objects like machines, furniture or the like. It is sometimes used to describe performance or service as well. I highly suspect that this word comes from Hindi because I couldn't find it in Persian Loghatnameh and I don't think it is a mispronunciation of any English word. So does such a word or similar one exist in Hindi? If yes then can you tell its original meaning please?

Yes, kachra / kachraa / kachda is a word in Hindi and it means "garbage / trash" and it is used for anything of poor quality also. 

word reference: कचड़ा, कचरा 

Hindi: Scandal

How would you translate into Hindi, "She is always surrounded by scandals"?

"Wah hamesha apwaadon se ghiri rahti hai." वह हमेशा अपवादों से घिरी रहती है.

Or even "Uska jeevan kalankon se bhara hai" उसका जीवन कलंकों से भरा है.

word reference: scandal

Hindi: Breakthrough

What will be translation of "breakthrough" into Hindi?

Breakthrough can be called safaltaa (सफलता); Major breakthrough = Badi safalta (बड़ी सफलता) or also bhedan (भेदन) and mahatvapurn khoj (महत्वपूर्ण खोज)

word reference: breakthrough, सफलत, hindi

Hindi: Word for Alter Ego

What would be Hindi term for "alter ego"? 

I am getting these two meanings from online dictionaries and these look fitting: 

Antarang Mitr अन्तरंग मित्र (Link)

Apar swaroop अपर स्वरुप (Link)

word reference: hindi, alter ego

Hindi: merii baat kaa buraa mat manaanaa

In the statement "<erii baat kaa buraa nahiiN manaanaa", can we replace "mat" with "nahiiN" in Hindi?

Yes, in Hindi mat मत and nahiiN नहीं are same.

word reference: merii baat kaa buraa mat manaanaa, मत, नहीं

Hindi: It can't rain all the time

Can you write this in Hindi and in Dev Nagri Script also? " It can't rain all the time!"

=> Har samay barish nahi ho sakti! / Hamesha barish nahi ho sakti!

हर समय बारिश नहीं हो सकती! / हमेशा बारिश नहीं हो सकती!

word reference: It can't rain all the time

Hindi: Are you surprised?

Can you translate into Hindi: "Are you surprised?"

"Kya aap ashcharya chakit hain"?क्या आप आश्चर्य चकित हैं?

Hindi: moderator

What to call "moderator" in Hindi?

Pariniyaamak (परिनियामक) or Sanchalak (संचालक)

else there are many options available in dictionary [Link] and [Link2]

Hindi : to dip

How can you call "to dip" in Hindi? As in "Is it safe to dip it in the tea?"

"to dip" => dubonaa डुबोना

"Is it safe to dip it in the tea?" => Kya ise chai me dubona surakshit hai? क्या इसे चाय में डुबोना सुरक्षित है?

Hindi: rakh rakhao

What is "rakh rakhao" in Hindi?

Rakh-rakhav (रखरखाव) means "maintenance".