PD-47: add lang prop to booking request documentation

[jamie6king]

What does this PR do? (please provide any background)

https://holidayextras.jira.com/browse/PD-47

• Added lang prop documentation to all EU products (tested that the prop worked & correct lang was being passed back from the API).
  • The prop doesn't appear to be required and defaults to DE, so updated wording to reflect that. (although happy to be corrected here :) )
• Added OpenSSL 1.1 install step as this not being installed caused weird errors when compiling dependencies.
• Slightly tweaked the whitespace so the table is aligned again.

Any tech debt?

None.

Screenshots / Screencast

• I have checked our general contributing document and the project specific contributing document (if present) and I'm happy for this to be reviewed.

By approving a review you are confirming you have...

• Witnessed the work behaving as expected (this could be on the author's machine or screencast).
• Checked for coding anti-patterns.
• Checked for appropriate test coverage.
• Checked all the tests are passing.

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[david-r-edgar]

Actually we probably support pl too now? Might be worth updating all lists to include that?
(And ideally doing some tests too.)

[jamie6king]

Actually we probably support pl too now? Might be worth updating all lists to include that? (And ideally doing some tests too.)

Just checked and yeah we do, added it to the list :)

(booking was successful with both a lounge & carpark)