Thank you for your question.
As explained here, one way to provide the localization strings for our controls is to use localization managers. This means that in order to actually use the localized strings that you provided, you need to set the localization manager for the Input assembly (where RadTimeSpanPicker resides) and use the resource manager that contains your localized strings. Here's the example, if you added the strings to the default resource file AppResources:
InputLocalizationManager.Instance.ResourceManager = AppResources.ResourceManager;
Let me know if you need further assistance.
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