Just came across this dataset. Looks cool.
One suggestion, though: Since the whole point of the dataset seems to be highlighting language change over time, wouldn't it be more transparent to list doculects as rows in LanguageTable, rather than Glottolog languages? This would get across the point that languages at different points in time might need to be treated as different more thoroughly - while aggregation on Glottolog language level would still be possible via the glottocode property.