Hi @Martin Kysílka and welcome to the Community!
From the tags you added to your question, I am assuming that your question is about changing the default language of the portal for your customers. If so, this support article lists the most important things to keep in mind.
In short, the built-in language packages will automatically translate most of the text that is part of the product itself. However, for custom configuration you added yourself, translations need to be in place. The translations feature mentioned in the linked article will help you find items that have missing translations.
A caveat to keep in mind: some items can't be translated. If you have custom fields on your screens that are dropdowns with predefined options, translating those is not supported.
Before changing the default language, you can always check what the portal would look like by changing your personal language settings to English and (pre)view the portal as such to get an idea of any gaps. Check out this support article to find how you can do so.
Hope this helps!
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