I am not a native English speaker. In a scientific paper I'm writing, I refer to geographical areas in Belgium: Kempen, Hageland. In Dutch, an article is used when referring to these areas: "in de Kempen", "in het Hageland". Is the article needed in English, or would one simply write: "in Kempen", "in Hageland"?
The Campine and The Hageland are the two English names you are looking for.
Note that English is not consistent with the use of the definite article when it comes to place names. Those which include a description of the type of geographic area or political entity tend to use the definite article ("The United ...", "The Republic of ...", "The ... Region", "The ... Republic"). Those that refer to historical or informally-defined regions that don't exist as modern political units are more likely to (The Burren, and indeed, The Campine and The Hageland), but may not.
There are exceptions in both directions, and also a tendency to move from names for countries that include the article to those that do not. This may have a political connotation, particularly given the tendency to use it for regions that are not political units, since there may be disagreements about whether a region should be a political unit. Hence for example the Ukraine government preferring that the still-common The Ukraine not be used, but Ukraine be used instead.
As such, it is in English important in learning the proper noun used of a geographical area, to learn it as including or not including the article, because there is no fully consistent way to know just by following rules.
According to Wikipedia:
Wikipedia does not suggest another spelling of Hageland but you still need the definite article the
The Hageland is a landscape in the Flemish Region of Belgium, situated in the eastern part of the Province of Flemish Brabant and extending into a western tip of the Province of Limburg. It is mainly comprised between the cities of Aarschot, Leuven, Tienen and Diest, and largely coincides with the historical County of Leuven. The French Government that controlled the area that later became Belgium in the last years of the 18th and early 19th century, had extended Limburg, which since then comprises the minor part of the Hageland at the city of Halen.
The name refers to land with dense (low) forest and/or undergrowth. Its earliest attestation (spelled as Hagelant) dates from 1528.