I have an ASCII system for writing sounds mapped to IPA letters, but I can't figure out what the "l" sound is as in Danish "sidde". It's pronounced sort of like (if I were to try to write it out) "sihlthle". What IPA symbol is that? I don't see it here, and after talking with a Danish speaker, they said it's a unique sound that wasn't represented by some other common IPA stuff I used (represented in an ascii system).
Wikipedia says "/ð/ – the so-called 'soft d' (Danish: blødt d) – is a velarized laminal alveolar approximant [ð̠˕ˠ]", citing Basbøll (2005), Grønnum (2003), and Ladefoged & Maddieson (1996).
This gets in to the difference between phonetic and phonological descriptions
The phoneme in question is generally called /ð/. The issue is that the precise phonetic details of how this phoneme is produced, differ quite a lot from prototypical [ð] (which is why English-speakers generally hear it as closer to our /l/ than our /ð/)
As Nardog says in their answer (citing Basbøll (2005), Grønnum (2003), and Ladefoged & Maddieson (1996) via wikipedia), this phoneme is pronounced as a velarised laminal alveolar approximant so if you need a precise phonetic transcription, [ð̠̻̞ˠ] would be appropriate
Note: wikipedia gives the phonetic transcription as [ð̠˕ˠ] (with no laminal diacritic, despite this being specified in the description of the sound in words, and also rendered with the lowered diacritic following rather than stacked)