One source of information is to try to find diaries from archives, both digital and from material that are archived but not digitized, such as papers and books at different libraries and similar.
One example is the diaries from Sven Blom who fought for the French Foreign Legion in the beginning of the war, around 1914-1915. He later got frostbites on one of his feets, and continued to keep contact with the other Swedes in the Legion through letters, from the hospital.
Some parts of the diaries are digitized, but there are a lot more to find at Uppsala University. Below a picture from Svens diary.
Source: https://ub.uu.se/finding-your-way-in-the-collections/selections-of-special-items-and-collections/sven-bloms-diaries/