Hashem doesn't refer only to God, it's used in Hebrew in the literal sense, meaning the name. "האם רשמת את השם שלך?" - Haim rashamt et hashem shelach? - did you wrote your name?
The comments makes me understand I was too brief, so I'll add a clarification:
In addition to God, it is forbidden to call parents by their name. Likewise, because of Kvod Malchut, it is forbidden to call a king by his name. These restrictions are not limited to calling them, it is also forbidden to reference them by their name. Instead, we call them in referencing terms with general meaning - mod, dad, the king. When used, all those terms refers to specific identity - the speaker's father or mother, the king who's identity is understood by the context. But while the reference in any specific use (speech act) refers to a specific identity, the meaning of the term is general.
Sometimes a term may change a meaning, like the term אל, El, which initially meant any god and so saying it was fine. Nowadays, the referencing of other gods is done almost exclusively by the term אליל and so it became forbidden to say אל when referring to God.
Regarding rachmana, I'm not sure. The word does literally mean something different in Aramic and I guess it was used in the past in the literal sense but it isn't anymore. I don't see it used to refer to God either, except within prayers.
