If I am not wrong, __MSG_ placeholders should automatically replaced in manifest and CSS files only, not in HTML files (other files like JS I did not test). In Firefox and Chrome such functionality must be added with own JS, and most developers added some extra functionality which is now broken.
Steps to reproduce:
Add "test" in localization file and __MSG_test_ _ (w/o extra space) in any HTML file.
__MSG_test_ _ should be not translated.
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