Update strings.txt with Danish language elements

[cfuttrup]

Hi Philippe. This is translated from English language, since only English was initially supported. Be aware that once I was unable to translate, so the EN is kept. Some places the EN version is equal to the DA version, so no translation. BE AWARE that line length is quite different in some places, I don't know if there is enough space. BE AWARE line breaks are different in some places, I don't know if the line break is important (I have assumed not).

[philippe44]

Thanks - You don't need to add anything when you don't have translation. EN will be used is nothing else is available. There should be no line break in strings.txt. If you want multiline in file for editing convenience (not in the UI), just have another consecutive line with the same language code.