How should "Northern Europe" be capitalized?

Europe should obviously be capitalized, since it is a proper noun. Should the northern part of the example sentence "I was traveling through northern Europe." be capitalized?

In country names such as North Holland, north is actually part of the name and not an adjective, and should therefore be capitalized.

However, in this case, there is no part of Europe officially named "Northern Europe", but is instead just used as a general (and varied, depending on you count them) list of countries roughly situated towards the north. What are the rules for capitalization in such a case?

The implied direction of your question is sound. As you say, "there is no part of Europe officially named "Northern Europe", but is instead just used as a general [...] list of countries roughly situated towards the north."

There is no need to capitalize 'northern' in this case, for the reason you implied. Contrast this with North Dakota or North Carolina (the official names of two of the United States), which do require capitalization of the "North" component, because that feature of the presentation of those names helps to mark them as official designations.

When 'north' or 'northern' is merely a general descriptor, no capitalization is required. Indeed, capitalizing the word would misleadingly imply that the term was an official name.

I therefore take issue with Translegal's guidance on this point.

Rules may vary between style guides, but the general principle is that if it is just a general region it will have a lower-case compass-point, whereas if it is a region specifically defined by officialdom or by an organisation whose definition is being used, it will have upper case.

The Guardian style guide recommends "north London, north Wales, north-west England, the north-west, etc". The Economist style guide says: "Lower case for east, west, north, south except when part of a name (North Korea, South Africa, West End) or part of a thinking group: the South, the Midwest, the West (in the United States, but lower case for vaguer areas such as the American north-east, north-west, south-east, south-west), the Highlands (of Scotland), the Midlands (of England)... Europe's divisions are no longer neatly political, and are now geographically imprecise, so use lower case for central, eastern and western Europe. But North, Central and South America are clearly defined areas, so should be given capitals as should Central, South, East and South-East Asia."

On this basis, "northern Spain" has no capital N, but "Northern Ireland" has one; and if we refer to "south-west England" in a vague sense, it shouldn't have capitals, but if we are referring to "South West England", one of the nine official regions of the UK, then it should be capitalised.

For most people (and presumably for The Economist), "northern Europe" isn't considered an officially defined area nor one with strict boundaries. However, EuroVoc, an official publication of the European Union, maintains an official definition of "Northern Europe". The United Nations also maintains an official (and different) definition of "Northern Europe". If you were using one of these definitions and thus using the term "Northern Europe" to refer to one of these specific, defined list of countries, then you would probably want to capitalise "Northern".