German, properly
Not a machine translation with English bolted underneath. The whole interface and all 157 exercise names are written in German.
Set your phone to German and StackUp is German: over 600 interface strings plus all 157 exercise names and the preset workout names. Bankdrücken, Latzug, Seitheben — the words people actually use in a German gym, not a dictionary's first guess.
Numbers behave the way German numbers behave. 42,5 kg with a comma, and typing it that way works — a tracker that insists on 42.5 is one you fight with every single set.
Some words stay English on purpose, because that is what gets said out loud: Workout, Split, Superset stays Supersatz but a Zirkel is a Zirkel. Translating everything to the last syllable produces German nobody speaks.
Search matches both languages whatever your phone is set to, so typing "bench press" on a German phone still finds Bankdrücken. Useful when the exercise you know from a video has an English name.
Try it
Free on Google Play. No ads, no account needed, and nothing to buy inside it.