A simple method from cryptanalys is frequency analysis. The method is calculating the relative frequency of the letters in a ciphertext. The you must compare that to the theoretical relative frequency of the letters in the language in which you assume the ciphertext is written.

Then you test decrypting. Iterate until the result is meaningful.

Given the message

Olq tlk kpn, tpa uhcu ly huklyz vn qln rhu nvka spkl rhmml. Qln nåy aps mvkivsk ocly apyzkhn hmalu vn qln ufkly kla.

And given the hint that the text is in Danish, try to decrypt it by means of frequency analysis.

Hint 1: Look up Danish letter and word frequencies including digrams and trigrams.

Hint 2: Try the character counting tool. Supposedly the best character counter.

Hint 3: Wiktionary:Frequency lists/Danish wordlist.

Hint 4: Dansk Center for Mumletekst.

Hint 5: https://www.dsn.dk/nyt/nyt-fra-sprognaevnet/numre/argang-1968-1984/marts-1970-pdf.

Hint 6: Relative frequencies of letters in other languages in the Wikipedia article Letter Frequency.

Hint 7: Danish Letter Frequencies.

Have a go now!