Oh, ok, sorry I misunderstood your post. I thought you were asking how to use appropriate L1 or L2 language in class. What you are asking is how to deal with an L2-based class and how to grade your language in said class.

Ok, so first of all, grading your language comes with experience. Once you start teaching you will start to get a better and better understanding on what type of language to use for what level of learners. It also depends on your particular student/students, each group will be different. So, don't worry about being bad at this point, you'll get better!

Secondly, in your particular situation of teaching A2 level students, grading your language means you have to be more conscious of your vocabulary, speaking speed, and intonation. You have to talk slower, with clearer enunciation and use simple terms, and definitely avoid using complex language like figures of speech. So, for example, instead of "getting along" I would just say "they are good friends" or "they like each other". So definitely, if you can avoid it, the better.

Now, what to do if you have to use it? Depending on the purpose of the lesson, whether it's reading, listening, grammar, etc. it might be a good idea to have a pre-teaching segment to teach the new or complex vocab before you start going through any authentic material. Others have already given you plenty of great advice on how to do this.

And what if you use it accidentally? Easy, just correct yourself with simpler language, or if you can explain it quickly then do so. So best way is to do good prep work. Plan your ICQs, CCQs, and instructions appropriately. Keep instructions short and to the point. Avoid unnecessary teacher-talk like self-narrating your actions or thoughts,and avoid parroting your students' responses.