It's not very common to use al- productively in new words but there are quite a few fixed words that have it. Examples include almáttugur, omnipotent/all powerful, alvitur, omniscient/all knowing, etc.Since the meaning is different, which one is used (if any at all) depends on what it is, you want to say. As far as I can tell lang- can be used with any adjective (it can also be stacked: langlangbestur). Al- is uncommon and I think I would always rather use allra. There might be some adjectives where al- is better but I can‘t remember any.