I18N: Add i18n to `size_format()`.

Add translatable strings to the units of the `size_format()` function.

Props Rahe, audrasjb, ocean90.

Fixes #50194.


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git-svn-id: https://core.svn.wordpress.org/trunk@47821 1a063a9b-81f0-0310-95a4-ce76da25c4cd