Translating my custom theme, always translated version?

You are making wrong assumption here. define('WPLANG', ''); doesn’t mean “original locale of the string” or anything like that, it is taken literally as “no locale specified”.

And when it’s nothing WP proceeds to assume default locale, which is en_US. This is why as soon as you add English translations of string they are displayed for this case.

So to test your original Dutch strings properly you need to set WPLANG to respective Dutch locale.

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