Chore: add test for locales - to find problematic keys

[benkoshy]

Why this PR?

• When users make submissions for locales, they often cut and paste from other locales, and incorrectly apply the rules.
• It is currently quite cumbersome to manually check whether the rules have been currectly applied. I would prefer if there was a test to help aid in the discovery of discrepancies.

For example, I might declare that the Klingon language has the e.g. :east_slavic pluralization rule, but I might incorrectly label as per : other pluralization rule: e.g.

# :east_slavic pluralization (see https://github.com/ddnexus/pagy/blob/master/gem/lib/pagy/i18n.rb)
klingon:
  pagy:
    aria_label:
      nav: 
        wrong_key: "this key applies to another locale"
        wrong_key_2:  "this key applies to another locale"

# all of the above is a cut-and-paste from another locale
# it does not apply here

Right now you have to manually check. But these test will help capture such discrepancies, and tell you what should be applied instead.

ISSUES with the PR

• ta.yml and swa.yml are excluded from the checks
• it will not check if the .yml keys have been misapplied to the wrong label. in other words, manual checking is still required.