tests: Update month name check for Greek locale

Update the abbreviated month name in the test to match the actual translation. Otherwise the test fails with false positive.

Vocabulary:

July (nominative) - Greek: Ιούλιος (abbreviated: Ιούλ) Of July (genitive) - Greek: Ιουλίου (abbreviated: Ιουλ)

This is similar to commit 4d215e00 and commit 7fe793e1.

Closes #1776 (closed)