You can check the content of your exported package (it might be a problem with export not import).
This error can be caused by:
- missing page item in the package (there must be at least one index.html file)
- First selected index.html has no information about exported language.
Open your package and if there is a index.html make sure that in the body element in class attribute there is a language information
Example body tag:
<body class="
{71E0EFC2-CC97-4066-B15F-7644972C4EFC}
{en}
{FE5D7FDF-89C0-4D99-9AA3-B5FBD009C9F3}
[Ignore-This] sc81 preview default-device [Ignore-This-End] add-your-css-classes-here">