(a) Translation is done as usual by means of po files, you don't have to touch the provided dialogs. Use poedit to translate the strings in "vpsr.po" (folder "languages") to French and save as "vpsr.fr.po". If you want to share this work upload the translated file here so that I can add it to the library. The demo project (examples/demo) is quite advanced and applies translation as well. The strings of the demo project are translated in the demo.*.po files.
(b) Well, the Ini Storage was just an excercise for me to better understand the datastores, it shows that the datastores can be very very simple. For a dedicated database system such as PostgreSQL you better use the SqliteDatastore or ZeosDataastore as a template. I'd guess PostgreSQL could be very similar. Unfortunately I do not have much experience with the big database systems, otherwise I'd already have written more dedicated datastores.
You can avoid writing a dedicated datastore by using the FlexDatastore. See the discussion in the other thread until a special example sample is available within the distribution.
There is also an SQLDatastore (vpsqlds) which appears to be very general and can be adapted by means of sqldialect units. But I though it would be easier to write dedicated datastores and did not yet look into this in detail.
I am not sure about missing fields, probably there will be crashes at least with the dedicated datastores. FlexDatastore might work because there is a separate assignment of database fields to internal fields. Once I'll be through with the debugging (there are still lots of bugs...) I'll have a look at missing fields. I think it shouldn't be too difficult: it would be sufficient to check the existence of fields on TCustomDBDatastore (unit vpdbds) which is the ancestor of all database-related datastores.
(c) The datastore has a property CategoryColorMap in which you can define the descriptions and colors of the 10 predefined categories (so far, it is not possible to change this number). Have a look at examples/demo, it customizes the first two categories.
(d) No, tvplanit cannot do Gantt charts. But since all data are stored in an ordinary database, nothing prevents you from exporting the data to special Gantt tools or fpspreadsheet. The Sqlite and ZeosDatastore expose the internal datasets as public properties.
Maybe I should mention that I have some plans for a Gantt series in TAChart in the future.
[EDIT]
I forgot to mention that the sqlite3 and zeos datastores are in separate packages, laz_visualplanit_sqlite3.lpk and laz_visualplanit_zeos.lpk because the required database components are not part of the standard Lazarus distribution.