I some weeks ago tried to convert a Drupal 7 site to Backdrop. As far as I know I followed the guidelines, but it was a disastrous waste of time.
I migrated the same site to D8 with hardly any problems, but I do not like D8. Several modules are not quite functioning (yet) and the whole construct is not to my liking.
I then tried to get into this forum for help, but it took more than two weeks to get approved - ridiculous. In that time I of course searched for other solutions, so, for me, Backdrop is not at all promising. Sad.
(I have several D7 sites in need of conversion before EOL (november 2021))
Hello again Egmund,
The instructions to upgrade a web site form Drupal 7 to Backdrop are still on https://backdropcms.org/upgrade-from-drupal, and they have been updated since our conversation, see https://github.com/backdrop-ops/backdropcms.org/issues/509. In case something is still misleading, feel free to post your thoughts here or in the mentioned issue.
Also, recently there was a Backdrop Mini Camp with a session called "Upgrading from Drupal to Backdrop CMS" which was recorded, see https://www.youtube.com/watch?v=SdHzcX947Vc.
Apart of that, I think possible upgrade challenges depend on the kind of site. If something doesn't work, don't hesitate to ask here or in the Gitter LiveChat or to post issues in the Backdrop core issue queue (see https://backdropcms.org/support/resources).
Regarding translations, I'll create a Danish translation group on https://localize.backdropcms.org and import some Danish language files from Drupal (core and key modules like Views). You will then be able to download the Danish language file from the Backdrop localization server and use it in your Backdrop sites. As a group member, you'll also be able to provide translation suggestions.