
undeutlich
unclear
Difficult to understand acoustically because someone is speaking quietly, unclearly, or mumbling.
Er murmelte eine undeutliche Entschuldigung und ging schnell weiter.
He mumbled an unclear apology and quickly walked on.
unclear
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unclear
Difficult to understand acoustically because someone is speaking quietly, unclearly, or mumbling.
Er murmelte eine undeutliche Entschuldigung und ging schnell weiter.
He mumbled an unclear apology and quickly walked on.

indistinct
Not visually clear to recognize; blurred, out of focus, or faded.
Durch den Nebel war die Küste nur als undeutliche Linie am Horizont zu sehen.
Through the fog, the coast could only be seen as an indistinct line on the horizon.

vague
Not precise or clear in meaning; vague or imprecise.
Seine Antwort war so undeutlich, dass wir immer noch nicht wussten, was er vorhat.
His answer was so vague that we still didn't know what he was planning.
The word consists of the prefix 'un-' and the adjective 'deutlich' (clear). The prefix 'un-' negates the meaning of the base word. So 'undeutlich' means 'not clear' or 'not distinct'.
Seine Handschrift ist so undeutlich, dass ich sie kaum lesen kann.
His handwriting is so unclear that I can hardly read it.
The context tells you what kind of lack of clarity is meant. In connection with 'sprechen' (to speak) or 'Stimme' (voice), it is acoustic. With 'Schrift' (writing) or 'Foto' (photo), it is visual. With 'Antwort' (answer) or 'Idee' (idea), it is vague in content.
Er sprach so undeutlich, dass ich ihn bat, es zu wiederholen.
He spoke so unclearly that I asked him to repeat it.
When it is about acoustic lack of clarity, 'undeutlich' (unclear) is often combined with verbs of speaking, for example 'undeutlich sprechen' (to speak unclearly), 'undeutlich murmeln' (to mumble unclearly), or 'undeutlich nuscheln' (to mumble/slur unclearly).
Der Patient murmelte etwas Undeutliches, bevor er einschlief.
The patient mumbled something unclear before falling asleep.
In a visual context, 'undeutlich' often describes things that you cannot recognize well. Typical nouns are 'Schrift' (writing), 'Zeichnung' (drawing), 'Kontur' (outline), or 'Bild' (image/picture).
Am Horizont waren die undeutlichen Umrisse von Bergen zu erkennen.
The indistinct outlines of mountains could be seen on the horizon.
unclear
Both words can describe a statement that is difficult to understand. „Undeutlich“ is especially appropriate when the voice or wording itself is weak and difficult to perceive. „Unklar“ is more appropriate when the content does not provide a clear answer or explanation.

Die Ansage war undeutlich, weil man die leise Stimme im Lärm kaum verstand.
The announcement was indistinct because the quiet voice was barely audible over the noise.
Die Ansage blieb unklar, weil sie keine genaue Uhrzeit nannte.
The announcement remained unclear because it did not give a specific time.