Starting with NAV 2015, if you import a language layer, you need to have a service instance running for this database. Up to NAV 2013, this was not necessary, and in NAV 2013 R2 you could set the "Prevent Data Loss From Table Changes" switch (that was removed in NAV 2015) to "No" in the options menu to do this without a service.
Answers
forum.mibuso.com/discussion/59251/the-table-definition-is-invalid-and-cannot-be-validated
beginnersnav.blogspot.in/2015/12/the-table-definition-is-invalid-and-can.html
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