Reza Putra's letter Non-Muslims too can reply to 'Assalamualaikum' is a welcome diversion from the debate upon who may rightfully use the word Allah to refer to their God.
Language and religion are, even today, closely intertwined. In times past it was more likely that the religion one professed would match the language of the surrounding community. Over centuries, religion has spread faster than the language itself, resulting in many countries which have never used Latin or Arabic using them in their Christian and Muslim religious services respectively.
When Reza Putra enjoins non-Muslim persons to say 'Assalamualaikum', he may have blurred the edges of religious language and common speech. I fear he would not be agreeable to using the Jewish and Hebrew greeting 'Shalom aleichem' and its proper response 'Aleichem shalom' or the Catholic Latin greeting 'Pax vobiscum' and response 'Et cum spiritu tuo' despite them all having the same translation as the Arabic phrase.
In just the same way, I feel that 'Allah' is probably not the right word for a non-Arabic Christian to use to describe his God. A more fitting word would be the Latin 'Deus', the Hebrew 'El'ah' or in the language of Jesus Himself, the Aramaic 'Elh'.
Both the latter share a common root with the word Allah but are more appropriate in terms of the religion and in terms of avoiding confusion.