I have seen G-d in many English translations of a siddur, bencher, or other religious text but have trouble understanding why. The best answer I have received is that if God is recognized by another commonly used name, even if it is not in Hebrew, you should not write it fully. This was very confusing for me because God is known by many names in English such as Heaven, Providence, Lord, He/Him and others. Even in the translations that had G-d Lord was not hyphenated. God is also known very widely in Hebrew as Hashem, which is usually abbreviated in Hebrew, but not in English transliteration or speech. I do not know for sure, but I also doubt that Allah is hyphenated or abbreviated in when a Jew is referring to God in Arabic. Why do some people write G-d and why are the other common names of God ignored in this respect?
When we wrote the soc.culture.jewish FAQ we dealt with this question as follows. Note that one of the students who had been in the class at the time that Rav Soloveitchik wrote the name "God" and erased it verified that the story is true.
Writing: Why do some people write "G-d" with a hyphen instead of an `o'?
Based on the words in Deut. 12:3-4, the Rabbis deduced that it is forbidden to erase the name of G-d from a written document. Since any paper upon which G-d's name was written might be discarded and thus "erased", the Rabbis forbade explicitly writing the name of G-d, except in Holy Books, with provisions for the proper disposal of such books.
According to Jewish Folklore, G-d has 70 names. However, only one of these names is the ineffable name, which cannot be erased or pronounced. Further, of the 70 names, seven may not be erased but they can be pronounced on certain occasions (such as when reading the Torah). The other names may be erased and pronounced, but still must be treated with respect. The Talmud (Shevuot 35a-b) makes it clear that this prohibition applies only to seven Biblical names of G-d and not to other names or attributes of G-d, which may be freely written. The prohibition was later codified by Maimonides (Mishneh Torah, Yesodei HaTorah 6:1-2). The practice of writing "G-d" is supported in Shut Achiezer, 3:32, end, where it is endorsed and accepted as the prevailing custom. Rambam cites Deut. 12-03:04, which states "and you shall destroy the names of pagan gods from their places. You shall not do similarly to G-d your Lord." The intent of this is to create an atmosphere of respect for G-d's name vs pagan gods names.
As a result of this, people acquired the habit of not writing the full name down in the first place. Strictly speaking, this only applies to Hebrew on a permanent medium, but many people are careful beyond the minimum, and have applied it to non-Hebrew languages. Hence, "G-d". One explanation is that using G-d is a reminder that anything which we may say about G-d is necessarily metaphorical. Spelling out the Name (even in a language other than Hebrew) would imply that one could speak meaningfully (not just metaphorically) about G-d.
However, the Shach (Yoreh De'a 179:11) ruled that "God" spelled in a foreign language does NOT have the status of a "shem" and thus may be erased, lehatkhila. There is a story about Rav Soloveitchik (z"l) intentionally writing GOD on the board while teaching a class and then just as deliberately and intentionally erasing it, so as to demonstrate by his own example that this was not a halakhically a problem.