recommandation plz

I suggest that you do a requirements definition; in other words, analyze everything that the new system needs to do. Don't write it in terms of how to do it as a developer; do write the requirements definition from the point of view of the user and describe what needs to be done from the point of view of the user. Do describe what the data is, such as what forms that the user will see and use. Describe what data needs to be where. Describe what data needs to be processed and what the processing (the events and actions and such) will be. Don't describe what needs to be done in terms of specific software.
 
Businesses usualy have software that they are already familiar with and using. It is valid to specify in a requirements definition that that software should be used or must be used. The requirements definition should however provide the necessary information for explaining why existing software is deficient, if it is, for the system.

Then sorry, until you find out these things I cannot suggest which language to use C# or C++.

How will you synchronize the central database from the satellites? (the distributed databases)

Depends on where the database will be housed and what do you mean by distributed database.