L10n: Use an absolute path for the `is_dir()` check when looking for installed translations.

Avoids warnings when `open_basedir` restrictions are in effect.

Props Profforg.
Fixes #34526.
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git-svn-id: https://core.svn.wordpress.org/trunk@35820 1a063a9b-81f0-0310-95a4-ce76da25c4cd