I know the word Allah in Arabic implies that there is only one. That's because Allah is either a name or a short form of Al Illah, literally meaning "the God". That depends on which muslim you ask, actually.
What about the English word God or god?
We got Zeus, a god, Hermes, a god, and then we got this Judeo-Christian God.
Does the word god in English imply that there is only one such being?
This post on the Biblical Hermeneutics sister site claims that it does.
In Islam, Christianity and Judaism God refers to the supreme being who is the creator of the universe. In other religions, the word god, small case, refers to a deity, a superhuman being, or something that is supernatural. So if you are in the realm of the three monotheistic religions, God is the right word to use for the creator of the universe. If you are discussing it in some other religions, then the small case god maybe used.
The word god does not imply anything about the uniqueness of a god in English. In religion however, it is a different matter.
It is common practice to write the word with a capital when referring to the Judeo-Christian one supreme being, so when you see it written as God, you can quite safely assume that the one god, supreme being and creator is meant.
When referring to other gods, the capital is not used, and it is understood that the gods referred to are usually not seen as the only ones of their kind.
An interesting third option is to capitalize the whole word, GOD or G.O.D., which I have seen used by some Hindus when speaking English to refer to the trimurti, the Hindu trinity of Brahma, Vishnu, Shiva to represent the creation, maintenance and destruction aspects of the one divine overlord.
Your question about the word in English seems a bit off-topic, as it is not the language but the religion that defines the properties of god or gods in that religion. Believers will adapt to their theological ideas to different languages. The word Allah is used by Christians in Arabic countries to refer to their own one God, but in India, Christians sometimes refer to God as Om, which is originally a Hindu concept with a different meaning.
"God" is generally understood by English speakers as the proper name for the Judeo-Christian god unless context suggests otherwise.
Little-g "god" in English is a word that implies "supreme being"; it is also singular. "Supreme" is a superlative. Logically, that implies just one.
Words have contexts, of course. In certain contexts "god" might imply a class of supernatural beings, so there may well be more that one. This is normally signalled, however, by qualifiers (e.g. "a nature god").
I leave aside what would seem to be metaphorical usages ("rock god!").
It should depends also on who is using it. When a non-believer refers to the god believed by Christians, he can still write him a god instead of God because it doesn't makes sense for a non-believer to call any god with a capital G.
