We recommend using the more descriptive phrase "long-term disability" only because the English term "disability" is rather vague and the German word "Invalidität" denotes the longer duration of disability. We also suggest staying away from a literal translation of "Invalidität" - namely, "invalidity (the condition of being an invalid) - only because that term is polysemous and has a very specific second meaning under the law, which could cause a problem in certain contexts.
There are multiple definitions - click on the translation that matches your context.
severe disability means:
an illness, injury, or condition that makes it difficult for someone to do the things that other people do
A £6m centre for people with learning disabilities has been opened on South Tyneside.
a disadvantage or handicap, particularly one imposed or recognized by the law
The suspension of his driving license, a major civil disability in his profession, resulted from his conviction for driving under the influence of alcohol.