Like you properly pointed out T LString is not a supported type for dynamic fields in Sitefinity, yet. We aim at extending the custom fields to support multilingual content with our upcoming releases this year. At this stage we still haven't announced the feature in our roadmap, but we will do our best to include it in the second half of the year. In the meantime what you can do is make a separate field for each language as a workaround.
the Telerik team
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