I completely agree that it is no major bug. However, the German translation has a typo at least. Should be "Download-Details" instead of "Download details".
I found out that this already exists in JD 3.9.
In transifex 3.9 it seems to be correct, see also screenshot:
en-GB "Download details" => de-DE "Download-Details"
However, in the translation of the deployment, it seems to be wrong:
line 80 of: com_jdownloads_beta_4_0_16\com_jdownloads\site\language\de-DE\de-DE.com_jdownloads.ini
COM_JDOWNLOADS_FRONTEND_SUBTITLE_OVER_DETAIL="Download details"
With inspection by the browser developer tools and a search in the PHP code, I found it the related code here:
line 1043 of: \com_jdownloads_beta_4_0_16\com_jdownloads\admin\src\Helper\StandardLayouts.php
$details_subheader = '<div class="jd_cat_subheader" style="font-weight: bold;">{detail_title}</div><!-- Details layout subheader -->';