Adjust locale button style on hover

[katina-anachkova]

Closes #1909

Motivation and context

Styling of the Locale button in the navigation not adjusted for mobile version - it is the same as the desktop version, while the other tabs` styles differentiate.

Styles of all MobileNav list items must be unified.

To solve that, i added sx property on the Locale button to style it for breakpoints below 900px

Screenshots:

Before	After

Testing

Steps to test

1. Resize the window to width below 900px, which is the breakpoint where the navigation switches mobile/desktop.
2. Hover over the Locale button - it should have the blue color that appears on hovering register/login buttons and transparent background.
3. The new style shouldn't affect the button with breakpoints above 900px.

[github-actions]

✅ Tests will run for this PR. Once they succeed it can be merged.