Why is the written language inconsistent throughout One Piece? I noticed even in the Englished Dubbed version of One Piece many of the props (flags, signs, etc) use Japanese symbols rather than alphabetical words, and now, watching on the Subbed version, I notice some things are actually written in English. For example an island entrance says "Punk Hazard" and "Danger" in English, even though they're speaking Japanese. 

I've noticed this kind of inconsistency elsewhere too, for example, even in the Subbed version, you hear many English words and names, such as their shipwright saying "Super!". I've nopticed more things like this throughout the entire series, having watched many episodes in Dubbed and Subbed versions.

So, is there an explanation for why English and Japanese are intermingled so heavily in One Piece? it seems odd for the text to be inconsistent. Maybe they produced a few different versions with different text forms, but that would be odd since it's not even an English dubbed version in some cases.  

Was the writer bilingual? Or was it perhaps a strategy to please English and Japanese viewers equally? Or some other reason?
Ashishgup mentioned general aspects of English in Japanese anime and conversely in his comment.

Beside that, note that English is an obvious feature of One Piece. The name of the manga itself is "One Piece" in Japanese, not ワンピース ! 

Looking at the Japanese version of the manga, you can quickly notice that shop signs, some brands, city signs and even some fight moves are written in English.  All important characters are introduced with their name dubbed in English in the background.

It seems then consistent that the anime keeps this feature (the anime being derived from the manga and not the other way around).

As for why it is so: mixing languages is more than ever a trend in all languages. Some mangaka will do it more than others. Eiichiro Oda does it quite a lot.