L1 in your lessons
Why avoid using any intermediary language or minimize its use as much as possible? From the very beginning of the course (i.e. A0 level) it is important that the student sticks to the target language as much as possible without translating.
General principles
any contact with a language other than the target language takes the learner "out of the world" of the target language and disrupts concentration
although working only in the target language is challenging for students, it prepares them for real-life situations in the foreign language world (e.g. the student must be able to say that they don't understand, they need to be familiar with such a situation so that it does not derail them) - if the teacher uses the L1 or a different intermediary language, they will never fully prepare their students for that
Language specifics
only a relatively small percentage of the vocabulary has word-for-word equivalents in both languages; in other cases, despite the overlap, the terms have broader (or narrower) meanings in the different languages and are used in potentially (and sometimes diametrically) different contexts
in terms of grammatical structures, even structurally very close languages differ from each other, and although it may seem effective in the short term, translation distorts the perception of the target language in the long term - the learner doesn't get a chance to detach from the intermediary language and does not perceive the world of the target language as distinct
„Slavic-Slavic“ lessons
Czech, Ukrainian and Russian (not all speakers of Ukrainian come from Ukraine) are structurally very close to each other and students will not encounter fundamentally new concepts (gender and declension of nouns, verb finality, etc.), the clarification of which in the intermediary language may be evaluated as beneficial despite the above points if the concepts are not present in the student's first language and are completely new to them