If you click on the gear icon and go to Advanced Build Options, you can turn on XeLaTeX.

It's also worth noting that you can use accented characters with pdflatex: adding \usepackage[utf8x]{inputenc} at the top of your preamble will usually do the job.

If none of that helps, feel free to send us the link to the doc via https://www.writelatex.com/contact and we will have a look.

EDIT: english

I personally hold the opinion that XeLaTeX should be the default anyway, e.g. Texpad app (iOS/Mac) already does this. I don't know of any downsides to choosing XeLaTeX over LaTeX.

I'd also like to see XeLaTeX become the default, but unfortunately there are still a lot of old documents and journal templates that are not compatible.

In the mean time, we try to auto-detect which engine to use, so if you load the fontspec package, for example, we'll automatically use XeLaTeX.

Why do you prefer xelatex to lualatex?

Simply never used lualatex. Should I be?

Thanks a bunch.