Quick answer: Localized builds have their own bug classes (text overflow, RTL layout, font glyph). Per-locale bashes surface them; English-only bashes miss them.
Localization bugs are real and specific. Test in the languages you ship.
Per-locale tester
One native speaker per shipped language. They play in their language; specific bugs surface.
Layout audit
Each UI screen, each language. Text overflow, button truncation, font glyph issues.
RTL layout
Arabic, Hebrew. RTL-specific bugs; usually structural.
Cultural review
Some content is culturally sensitive. Native speakers flag; can ship adjusted.
“Localization is more than translation. Test in the cultural context.”
If you ship localized, the per-locale bash is non-optional. Quarterly cadence at minimum.