xDB Cloud Service 1.0 and 2.0 do not support custom cultures that are used for adding new languages to on-premises Sitecore XP instances, like Content Management and Content Delivery. As a result, the aggregation process can be broken after deploying Marketing Definitions (Campaigns, Funnels, Goals, Marketing assets, Outcomes, and Taxonomies) or after tracking interactions where a visitor's language is based on an unsupported culture.
xDB Reporting and xDB Processing services are based on Windows Server 2012 R2, which supports a limited list of cultures out of the box. Therefore, you must add languages to Sitecore XP using only cultures that are available in Windows Server 2012 R2.
Note: For the full list of supported cultures in Windows Server 2012 R2, please see the table with Release Key "8.1" and earlier in the following article: https://msdn.microsoft.com/en-us/library/cc233982.aspx.