Plain language and SEO

Hi,

I am currently working with an NGO that provides psycological counseling
for refugees in germany.

Due to the mixed language skill of the target audience, we decided to
provide the website in plain german as well as some common languages
like arabic and english.

As the organisation depends on voluntary work, there is a second target
group of skilled professionals who will use a very different set of
search terms.

So my question is this: How can we write the contents in plain german
and still ensure that they can be found via techical terms?

tobias