How to change excel sheet language settings?

"German english" - that is funny. You mean "Germish"?

Formulas are in the language of the Excel application installation, and you can't change that. If you load a german workbook into a UK Excel, all formulas should be seen as translated.

https://support.office.com/en-us/article/Translate-text-in-a-different-language-287380e4-a56c-48a1-9977-f2dca89ce93f

If you mean to change the keyboards language then

Navigate to Control Panel-->Clock, Region and Language-->Region and Language--> Then go to Keyboards and Languages--> Change Keyboards then select your prefered language from dropdown list.

sorry, number  (12,666)  format is in German . How do i change this to US format

Navigate to File--> Options->Advanced, in the first group "Editing options"
- Uncheck Use system separators.

I have unchecked the system separators but still showing the separators  in the sheet. How do i remove separators  from existing sheet ?

Are the cells numbers or strings? You should see if you edit a cell.

Navigate to Control Panel-->Clock, Region and Language-->Region and Language--> Then go to Formats Tab and select the country that you want to use format style. Also click additional setting to format numbers.

I have edited the cell its in number format.

number 1 is showing in in this format 1,000. How do i remove   ,   and  0

Your OS is having which regional settings? And are they active in Excel (should be in opposition to http:#a41387015)?

The number you have provided "1,000." is in-fact 1000 why do you want to change it to 1.

If you want to change it to 1 anyway then please use a formula to do so such as following.

=LEFT(D2,SEARCH("0",D2)-1)

or

 =SUBSTITUTE(D2,".",",")/1000

1,000 in German (and other) format is the same as US 1.000 - the value 1 with 3 decimals.

then why it has a "." after zeros?

Open your eyes and see - the number is "1,000", the dot you spot is part of the sentence.