**Question…do most Protestants use the German Bible? Was Luthur translating the implied meaning from Greek to German…or did he go back and add “alone” in Greek to his manuscript THEN translate it into German? **
The whole “Luther added “alone” to the Bible” has always been a strange and curious arguement to me…I’m not conversant in any way shape or fashion with German…since Luther didn’t translate into English…I have no idea even how to address the question.
Okay, being a Native Speaker of German and often using Luther’s Bible, I’ll try to answer your question here!

[Despite not being Lutheran!]
ALL Lutherans, in Germany as well as in Austria use Luther’s Bible. (in a modernized language; Version of 1986; unlike the KJV which still uses the English of the 17th Century).
In Austria the mixed (Reformed and Lutheran) Evangelical Churches use mainly Luther’s Bible. - Depending on which groups is larger, although up to 90 percent in every mixed confessional Church the Lutherans are larger here.
(The Reformed Evangelical Churches in Germany, Austria and Switzerland [where there are hardly Lutherans!] use the Zurich Translation of the Bible, which is without the deuterocanonicals.)
Sadly, I don’t know if the Lutherans here in Europe use Luther’s (German) Bible with or without deuteros. I only know that in Religious Instruction the one with deuteros is used.
But I am sure that the SELK (roughly the same as the LCMS in the US) use the one with deuteros.
You can buy in every shop here Luther’s Bible with or without deuteros.
In my Evangelical (Baptist) Church, some people use, besides other translations - one is free to choose which translation fits him personally best - , Luther’s Bible without deuterocanonical books. - The same goes for the other “reborn Christian Churches” here in Central Europe (A,D,CH).
I have a Luther’s Bible without and one with the deuterocanonical books. (The one with deuteros, was the first Bible I bought myself, shortly before I have enthrusted my life to Jesus.)
As Luther’s Bible only was modified some times so far regarding language, I think (and this goes to the OP) the content has never been changed. So yes, Lutherans still use the word ‘alone’ added after the word ‘faith’ in the following verses:
Romans 3:20; Romans 4:5; and Galatians 2:16.
Was Luthur translating the implied meaning from Greek to German…or did he go back and add “alone” in Greek to his manuscript THEN translate it into German?
Of course he interpreted, like everyone does when translating. So he “added” the word alone after the word meaning faith in Greek, because from the context it was clear that it was meant like this. Being (nearly - I haven’t finished my studies) a translator, I know what I am talking about.
Hope I could help you,