The reason this plural noun is translated into other languages as a singular noun is because it's being used with a singular verb.
This would be comparable to saying "Ants is here to stay" instead of "Ants are here to stay". It turns this plural word ("ants") into a proper noun.
Example with singular verb
In Genesis 1:1 (referenced in the question), we see a great example of the plural elohim being used with a singular verb:
In this text, elohim is being matched with the singular verb bra. This indicates elohim is the proper noun and it's translated as "God".
King James Version
In the beginning God created the heaven and the earth.
Example with plural verb
However, when Elohim is used with a plural verb, it's translated as plural "gods":
Here, elohim is used with olim, which is also plural. This is, therefore, translated as "gods" (or sometimes "spirits").
King James Translation (emphasis added)
And the king said unto her, Be not afraid: for what sawest thou? And the woman said unto Saul, I saw gods ascending out of the earth.
Plural as Singular
Dori pointed me to a question on Judaism.se discussing this point. Their answers show that there are some words in Hebrew that, although they end in the im (making them plural), are not actually plural. The Hebrew word for "sky" (shamayim) also ends in im, making it plural in some cases, singular in others: based on the verb. (Also, this makes the translation "heavens" or "sky" based on the plurality, for what that's worth.)
This accounts for why a word that appears to be plural--indeed many reference sites indicate that it indeed is plural--but is actually used as a singular word. When we read the original Hebrew, we presume that because it ends in im, it is plural. The truth is, though, that this may or may not be a plural word; we have to look at the verb.
(There are a couple of analogues to this in English: For example news, clothes, species, panties, etc.)
Because of the verb that is being used is singular, we can be confident in our assessment that this is a singular god that is performing all these tasks. Also, the use of the singular verb converts this plural noun into a proper name.