Hi from Paris,
I reply in English to be understood by most of our fellow detectives.
When persons lived in France for some decades, as Daniel Stenberg did, there is a high probability they became French citizens.
They had to give a lot of documents for their naturalization file : parents identity, list and identity of siblings (even living in foreign countries), places they lived since they left their bith country, places where they worked, they survived WWII, etc...
This file is in French National Archives near Paris and is accessible upon a special request on their web site
File is not available on line, but you can come to scan it on place or ask for a copy, but delays are in months.
These archives are still closed due to lockdown.
For persons who lived in Paris, you can get also data in Paris city Archives :
You must understand that French policy of privacy protection does'nt fully open all these archives online and it's much more difficult to recover our parents' past.
Archives and history of Medem Center - Arbeter Ring (Bund / Workmen circle) of France
Flam, Agid, Stoltz around Lemberg
Zysman, Kronenberg, Rottersman around Lodz