By default, a translator can translate any unlocked string in a resource they have access to (see how to lock strings).
If you want a translator to work on a specific set of strings, use the batch edit mode to tag a group of strings with the translator's name. Each translator can then find the strings they should work on by filtering for them, using the Tag filter and their name, e.g., TAG: John.
More information can be found here.
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