You can prevent the titles and captions of your works from being automatically translated using the following methods.

Applying the setting to all your works at once

If you turn off Automatic translation, automatic translations will no longer be displayed for the titles and captions of your existing or future works.

Any manual translations you've provided will continue to be displayed even if you turn off this setting.
*You can change the automatic translation settings for individual works from the posting and editing pages.

In the pixiv app, you can review and change this setting under Settings > Automatic translation on My Page.
Supported versions: iOS app version 8.10.0 or later, Android app version 6.200.0 or later.