testing: move l10n proposal to design directory

Change-Id: I54becec400c115d81bfd7091acba8b1576ffa468
Reviewed-on: https://go-review.googlesource.com/19011
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/design/text/12750-localization.md b/design/12750-localization.md
similarity index 100%
rename from design/text/12750-localization.md
rename to design/12750-localization.md