If taxonomy vocabulary changes from Translate to Localize how to combine terms?
Looks like you should consider using the Taxonomy Manager module. From its project page:

... provides a powerful interface for managing taxonomies. A vocabulary gets displayed in a dynamic tree view, where parent terms can be expanded to list their nested child terms or can be collapsed.
The Taxonomy Manager has following operations and key features:

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merging of terms (using the Term merge module in 7.x)
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