Full Idea
Tarski defined various predicates of the form 's is true in L', each applicable to a single language, but he failed to define a predicate of the form 's is true in L' for variable 'L'.
Gist of Idea
Tarski defined truth for particular languages, but didn't define it across languages
Source
comment on Alfred Tarski (The Concept of Truth for Formalized Languages [1933]) by Donald Davidson - Truth and Predication 1
Book Reference
Davidson,Donald: 'Truth and Predication' [Belknap Harvard 2005], p.15
A Reaction
You might say that no one defines 'tree' to be just 'in English', but we might define 'multiplies' to be in Peano Arithmetic. This indicates the limited and formal nature of what Tarski was trying to achieve.