ISO file size unit is not translatable on the download page

#7417 (closed) introduced the ISO file size on the download page, in a way that the unit (currently: MB) is not translatable. I didn’t see that as a blocker for merging, but still it would be nice to fix it.

Feature Branch: doc/9729-translatable-iso-size

Related issues

Related to #7417 (closed)
Related to #10124 (closed)
Related to #6907 (closed)

Original created by @intrigeri on 9279 (Redmine)

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