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With the current version of the API it is possible to store languages unknown to Umbrella Faces (example: "en" or "ru_RU").
With our next release a validation on the language will be implemented. As of then we accept by Umbrella Faces supported language codes only. There will be a new separate call available for API customers to fetch a list of supported language codes. Details and an example can be found within our Profiles on Demand API documentation: Profiles on Demand API Documentation Get language codes
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With the current version of the API adding an arranger and/or approver results in an error, if that persons profile has not been set as arranger and/or approver prior (error message example: "Could not find contact of type APPROVER with UUID xxxx-xxx-xxxxx-xx-xxxxx").
With our next release we are going to promote these persons profiles to arrangers and/or approvers automatically. This prevents profile updates from erroring out, and ensures consistency with existing handling for other connected systems (e.g. Concur).