JP/CN-only... it was recently trademarked in English in Japan, as "Natsu-Mon: 20th Century Summer Kid", so I guess it's just a matter of waiting on the global publisher to announce/finish the EN loc.
I know a lot of people weren't satisfied by the Shin-chan game and might be wary of jumping into another not-Bokunatsu so soon after that one but I can confidently state that this game is much truer to Bokunatsu, and that one shouldn't worry that it's going to checklist them to death just because it's "open-world".
Incidentally, the Switch version got some paid DLC today, and I do wonder how they'll handle that when the English version is released, and whether there might be any other loc-specific changes/additions in the works... any degree of character/gender selection would be welcome, for a start.
just saw they added a note to the Steam page: the English loc's being added on August 6!
(Reposting this because I deleted my original response on accident...)
I know I say this about this game literally every time it comes up on this site, but Natsumon is a super sweet and charming little open world Japanese summertime simulator. Just a ton of heart throughout the whole experience top to bottom. I can't vouch for the English localization, but given Spike Chunsoft's track record in recent years, it's probably perfectly fine at the very least, if not genuinely good.
I don't think this game is gonna sell particularly great overseas for a host of reasons, but if you like the sort of games I cover, sincerely, please consider giving it a chance. I have yet to finish it myself, but I poured a ton of time into it last summer myself and enjoyed every minute of it. It's another wonderful testament to the long-running Japanese industry trend of taking the open world format and applying it to different thematic and narrative ends than you'll find in western games and it's a genuinely touching game for it. 
