In the remaster, exposed strings are intended to be lookups into a the game's Localization Table. NL, appearing on strings that don't follow this convention, presumably stands for 'Not Localized'. If the string follows the convention of a lookup, but doesn't actually point to anything in the reference table, the string seen in game is instead replaced with '<MISSING STRING TABLE ENTRY>'. Since the old Construction Set is only fit to work with classic data, where strings just are as they appear in the editor, edits to existing data specify changes that result in strings being reverted in the resulting plugin. Entering new cells causes a crash. And modifying existing cells in general requires extra work, because like the problem with strings, CS is still only working with old data.
For adding new text and cells, the most lucrative solution I've found is MagicLoader, which enables modders to link everything up with a .json file, and have .pak files be generated with the appropriate patches.