Twenty Fifteen: use better, universal sans-serif fonts for RTL, similar to Twenty Ten which also had serif fonts throughout.

Props iamtakashi, see #30752.
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Update bundled theme POT files in trunk.

Built from https://develop.svn.wordpress.org/trunk@30965


git-svn-id: http://core.svn.wordpress.org/trunk@30952 1a063a9b-81f0-0310-95a4-ce76da25c4cd