Some comments about the previous suggestions. I worked on extracting coordinates and merging/picking best coordinates from the different languages.
In addition to wrong sign or easting/northing like pointed by Paul, frequent errors are due to copy/paste from other articles. Detecting outliers helps. Using a fixed radius often doesn't work well. A solution is to ignore coordinates where the sum of the distance to all others is significantly higher than the average.
A warning about Sven's idea to prefer local language Wikipedia. It works well in your example (Netherlands / nlwiki) because that is clearly the dominant language in this country, but in general it is not easy to determine "the local language":
- An article about a large entity may span many language regions
- The entity may be located at the border of such regions (take the Matterhorn peak, at the border of Italy and Switzerland)
- Countries or regions may have many official languages (e.g. Belgium, or region of Brussels)
- Or may have recognized regional languages (e.g. Spain: Spanish and Catalan). Determining a single local language is arbitrary in many cases.
Also, I found out that for languages where the wiki is rather small (in terms of number of articles, or unique authors), coordinates quality is usually not better than in large wikis (e.g. English). In a large wiki, the probability that somebody notices and fixes wrong coordinates is higher.
Another challenge when merging coordinates from all languages to Wikidata is that a single article can have several coordinates. E.g.:
- Main coordinates about the actual topic (usually with display=title, and shown top-right), and other coordinates not about the article topic itself but mentioned inline when referring to something else (the latter can be present alone too, without main coordinates). These in-lined coordinates should not be considered when trying to pick coordinates for Wikidata.
- Some templates require several coordinates, typically linear features such as rivers, that have source and mouth coordinates. Dealing with these cases is required for useful results.