Hello,
i understand your need for integration between your Salesforce instance and your Outlook inbox. Salesforce does provide tools for these. What i do not know is whether your instance of salesforce has those privileges or should be enabled by Salesforce. But we will see it once engaged on this project and received access to salesforce administrator account and your outlook.
Adding custom fields to the contact,account objects will be the next step and then, there should be modification of your layouts to represent your data.
Import, will be performed. Testing for consistency of the data.
Despite the system is in German and i am not German speaker i do not foresee huge difficulties since the number of words will be limited and those will be labels or settings, which i can easily accommodate to. Or in worst case ask some of my colleagues to translate. So it should not be a worry.
Expecting your further details.