As expert intercultural communicators, translators constantly face the challenges of comprehending and producing language that is stylistically appropriate in various communicative contexts. To scale these challenges, they must acquire advanced levels of sociolinguistic competence. Although sociolinguistic competence is considered an essential component of translation competence, to date no study has investigated how sociolinguistic competence, in the form of sensitivity to grammatical (in)formality, develops in translation trainees. Using style-based grammaticality judgement tasks, we collected data from 21 Dutch-speaking undergraduate trainees over a three-year period. We asked participants to revise sentences for style and investigated their accommodative competence in L2 English. We looked at participants' ability to accommodate language to social context through style-shifting, mapping how they detected and/or corrected (in)appropriateness in formal contexts. Our results show that trainees' overall accommodative competence initially improves, but subsequently stagnates. In the final year of testing, they barely score 50%. Receptive and productive sensitivity to grammatical (in)appropriateness follow similar developments, with trainees consistently performing better for receptive than for productive sensitivity. Our findings highlight the need to design effective sociolinguistically responsive (foreign-language) instruction in translation training to further develop sensitivity to grammatical (in)formality and to heighten sociolinguistic awareness and the controlled use of stylistic variation.