I have been wondering whether or not it is ok to use L1 in the classroom.

One of my teaching jobs is at a VHS (it's kind of a community college for those who don't know Germany), where the majority of Ss are middle-aged/'older' adults. The courses are very basic (Example course title - A1.1: mit Musse "At Leisure") with a mixture of REAL-starters and false-starters. Of course, the courses are monolingual and the school expects the teachers to use German in classes.

I try to keep my German use to an absolute minimum. If a student asks me a question in L1, then I always encourage them to ask me again and try to use English.
Ex from beginning of A1.2 lesson today (3rd session of a new course)....

Ss: Kannst du das an den Tafel schreiben?
Tt: Can you say that in English?
Ss: Write it!
Tt: Friendly? ....said with a :-)
Ss: Can you write that on the table, please?
Tt: Table? (touching a table)
Ss2: Board!
Ss: Can you write that on the board, please?
Tt: Perfect question! Yes, of course!

I also did a very quick drill of the Ss question.
Even though it looks long-winded when written out, this transaction only took a matter of seconds. I was pleased that 10 mins later, one of the lower level/confidence Ss used the same question absolutely perfectly.

That is just an example to illustrate how I always try to promote English usage above L1.

However, there are some occasions when a quick translation can save so much time and Ss frustration. I've been thinking that it would seem overly pedantic to insist on no L1 usage in these cases.
In another 'Mit Musse' A1.2 class last week, I was a bit annoyed that the coursebook CD suddenly threw up a 'Would you mind -ing...?' 'No, not at all'/'Sorry, I....' function.
Nothing wrong with that in principle, but this was a unit (and CD dialogue) based on 'Did....?'
Ex: Did you forget your friend's birthday? Yes, I did / No, I didn't

So, to have to deal with (what I would say is) the far more complex structure of, 'Would you mind -ing' and its seemingly negative response (No, not at all) for a positive answer was counter-productive, considering that we were dealing with something so basic as the 'Did...?' structure.
I attempted an explanation with 4 or 5 examples
Ex:
Tt: Would you mind sitting next to Martina?
Ss: No, not at all
Tt: Would you mind giving me your jacket until next Winter?
Ss: Sorry, I need it

...but there were still some blank faces and furrowed brows. I explained that this type of request is maybe overly formal and not so common, but I could still sense the Ss worry.

Next, I resorted to the German translation...
Tt: "Wären Sie so freundlich, die Tür aufzumachen?" 'Would you mind opening the door, please?

...and the penny finally dropped. Well actually, the rocks fell from the sky! The relief was as obvious as a hog roast at a Bar Mitzvah.

So, after being sold up the river by not entirely thought through textbook/CD materials, I saved a whole lot of duress by quickly codeswitching.

Is there anything wrong with that?

My own teaching blog is here.