Externalize ".. of .. checklist items completed" to make it translatable
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The following discussion from !107233 should be addressed:
- [ ] @mrincon started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/107233#note_1217665643):
> ...
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> Also, this string will fail in other languages, I am not sure if there may be an issue for this already?
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