Hi Yasushi
You want to display all site in english or just only the widget tag? Our wordpress default site is in english. I think you have installed the japanese language.
If you care good in coding then you can make child theme and change the language.
here is the link
https://codex.wordpress.org/Child_Themes
the code for widgets is available at
wp-content\interface-pro\library\widgets\interface_widgets.php
you can have a look here. Dont change here just make the child theme and change it. If you change here then while updating our latest version all data will be lost so you need to make the child theme. If you are not able to change then you may hire the developer to change it
Regards
Sunil
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