Hi Colum
Yes, the _LN tables are the language tables used for translation purposes. It stores the different data warehouse language variations for the investments, projects etc.
An example on how this would work:
DWH_INV_INVESTMENT_LN table will have 1 record per language for each record in DWH_INV_INVESTMENT.
Although currently you are only using the English language, the system supports you to add new languages at any time and this is why the tables are still present. If you think it would be best not to have them if using 1 language only, feel free to raise an Idea here in the Communities.
I hope this helps -Nika