I’ve been wanting something like this for a long time. Glad to have it!

Ondrej Jób made a wonderful work. Not only the information is deep and full of resources, but also the site works very well. This is one of the best contributions related to diacritical use and language support. It deserves to be widely known.

What I like about this is that through this work it has become rather easy to see how all these accents work in ‘real life’. On the other hand, I also found some things that are somewhat questionable (in my opinion, being Dutch).

Dutch language
jè
I am afraid that I’ve never seen that word. Google tells me that it may occur in ‘jè t’aime’, but that is a typo of ‘je t’aime’. Which may occur in Dutch texts, but ‘je t’aime’ is not Dutch, it is French for saying ‘I love you’.

chambéryzomer
That seems to be a typo. Could be Chambery zomer (‘Chambéry summer’, could be an offering of a travelling agent, alternatively to ‘Chambéry spring’. But that does not turn Chambéry into a Dutch word. Chambéry is a place in France.

eénvandaag
Similar to chambéryzomer. Could be ‘één vandaag’ (one for today). But that does not make it a sample for the occurrence of eén in Dutch. I have seen éen in Dutch but never eén.

matthäus
As far as I know, Matthäus is not an official Dutch word, it is the German way of writing Mattheus (the evangelist). Shure one can find ‘Matthäus’ in a Dutch text, but that alone does not make it a Dutch word.

köppen
My Dutch dictionary does not list that word and I would not know what ‘köppen’ might mean. Köppen might occur in Dutch text as the name of the German author who wrote several books on color models. But in such a case Köppen is written with a capital K.

luïciüs
Google tells me that this is probably derived from the family name ‘van Stipriaan Luïciüs’. I only found 9 entries with that name, they refer to two people, a father and his son who lived around 1900. How Dutch is that? Or more serious: Is that really relevant to the Dutch language?

sébastien
Not a Dutch word, but a French name: ‘Sébastien’. The Dutch would write ‘Sebastiaan’.

Anyway, I could not find mistakes like this in the German section, so maybe the quality of the Dutch section is just an exception. Still I hope that Ondrej will find some time to check and correct it.

Great work indeed.
BTW: In polish part: Kołłątaj, Bułharyn*, Maćkała and Idźkowski are family names, Łobżenica is a town.
* polonized Tatar name

A-J. wrote

I have seen éen in Dutch but never eén.

The capitalized version of één ('only one', emphatically) is Eén -- note the missing acute on the capital. Maybe Ondrej examined case-folded data, as evidenced by your other examples.

Nevertheless, very good work and a welcome reference!