Language is a means of expressing thought, and at the same time has the aspect that we use language to think.
Literal
Language [called-というの][topic-は] thought [object-を] express means [copula-である][simultaneous-と同時に] language [object-を] use-[and-て] think [quotative-という] aspect [subject-が] exists.
A philosophical observation about the dual nature of language. ~であると同時に 'while being X, at the same time' links two complementary descriptions. ~という側面がある 'has the aspect that' frames a claim cautiously. The sentence essentially describes the Sapir-Whorf hypothesis in lay terms.