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Hi Marten,
I have gone through all the posts.
For the requirement that you want to change the default language of the root site from English to Swedish, it is not possible. It depends on the country you chose when registering the tenant. The only workaround is to backup all the contents of the root site collection -> delete it -> recreate it from scratch. I believe you won't do it like this.
For the list template issue, the reason is that you create it in a Swedish site and want to use it in an English site. Please be informed that it is not supported, as SharePoint can't handle the list template between sites within different language. However, here is a workaround that we can change the language code in .stp file manually, then we can use it.
For details, refer to: Copy SharePoint Lists between sites with different language templates
The article is not very accurate. When using this article, please note
1.In extract the template file part and editing xml file, please search for 1053(Swedish) and replace all to 1033 (English)
2.When copying code to defination.ddf file, please replace 30000000.000 based on your own situation.
After you recreate the template, you can upload it to list gallary and you will be able to add it in add an app page.
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Best Regards,
Sky