The language does change when you change the language setting in Chrome's Settings (chrome://settings/ -> Advanced Settings / Languages). The problem seems to be specific to the Quick Language Switcher for Chrome and maybe other such plugins.

Reproduce the problem:

• Install any Locale changer extension for Chrome (or your browser), such as Quick Language Switcher for Chrome
• Switch to Spanish with the extension
• Go to https://ci.blueocean.io/
• Check that Jenkins is displayed in Spanish.
• Go to https://ci.blueocean.io/blue/pipelines
• BlueOcean is displayed in English instead of Spanish

Spanish translation is done in BlueOcean, so it should display BlueOcean pages in Spanish.