Keywords - in how many languages?

You only use the declared language because the keywords will get translated into any language, the potential customers are speaking. It is therefore important to stay in one language, and to spellcheck your keywords and titles.

As the titles do not get translated for the customer, I limit myself to English, but if you are unsure about your English, I suggest you use the language you are best in.

From https://helpx.adobe.com/stock/contributor/help/titles-and-keyword.html:

Keywords and titles need to be in a single language — the one you selected in the “I’m writing title & keywords in” dropdown menu in the Contributor portal. 

For effective and accurate keywords, we strongly recommend using the language you’re most comfortable using. The Adobe Stock search engine automatically translates your keywords when buyers search in English or their local languages.

We encourage using your local language for adding metadata to assets that have local relevancy, such as people, uniforms, traditional dresses, architecture, flora and fauna. It’s important to always verify the selected language in the “I’m writing title & keywords in” dropdown menu when you want to add the metadata in your local language only for a subset of your assets but not others. In case of a language mismatch between the menu selection and your metadata, your content won’t be surfaced in searches.

(bold and red is mine!)

Ideally, keywords should be entered in one language.

Let's say you've registered an asset in French.

If you search in English, the results will be translated from French to English.

If you enter keywords in multiple languages, the translation function will not work properly.

you can use two words such as "up high" not sure about more than that