The name of the Wither in German is Withern (should be of course Wither).
In crowdin.net the entry entity.WitherBoss.name is missing, so the translation cannot be fixed by the community.
In [MC-126] Nathan Adams wrote:
[Mojang] Dinnerbone (Nathan Adams) resolved MC-126 - Wither as 'Invalid'
Please fix (anyone can do it!) translation issues at http://crowdin.net/project/minecraft
We would like to do so, but since the entry is missing (see the attached screenshot), we cannot translate.
That's not a German-only issue, also in several other languages that entry is missing.
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@Selbram: Please read the new description: We cannot translate s/t in crowdin.net what's not there...

Nope, we just can translate the strings which Mojang has uploaded to crowdin.net.
I will tag Dinnerbone to take a look.
Nothing happened here 😞

@Meta: You should've better vote for this bug.
@Kumasasa: I already did ...

Update: entity.WitherBoss.name and potion.wither reappered magically in all languages at crowdin.net. Database error there ?
So that issue might be solved with one of the next snapshots.

Not fixed in 13w09a, even if the changelog states "Language files have been updated"
The entry is on crowdin, even if it hasn't been included in the game yet

Still "Withern" in 1.5

Confirm in 1.5.2

@Leon -> MC-14795
Why did you clone this? Dinnerbone gave you the tools you need to fix this.Better question, can you add the entry?