The application language of Family Historian is currently English only. By this we mean that the menus, screen labels, dialog boxes (window forms), and messages are all in English (American or UK English). However, the program is designed to be used to record genealogical data in most languages of the world. We call the language in which data is recorded, the project language. You don't need a language pack to record your data in English or in any other language. You only need them if you want reports, diagrams and other output to appear in any language other than English. We call the language of a report or diagram (or book, or any other form of output), the output language.
If the project language is not English - if, say, it's French - then with most reports, those parts of the report that simply output recorded data, will also be in French. But unless you use a language pack, the report title, the headings, the labels, and other details, will all be in English. For that matter, any dates, and ages, will all also be in English. To get the report to appear consistently in French, you will need to select a French language pack for that report. The same is true in diagrams. So that is one use for language packs: to ensure that the output language matches the project language, where the latter is not English. Where you use a language pack to ensure that the output language matches the project language, we call this same language rendering.
There is another use for language packs though, and that is to produce output in a different language to the project language. For example, if you have recorded your data in English and want to send a report or chart to a Norwegian relative, in Norwegian, you can use a language pack to do this. We call this other language translation. For that matter, if your project language is French, and you want to produce a report or chart in Norwegian, you can do that too. And that too is other language translation.
Not all reports and diagrams are 100% language pack ready. With those that are, however (which is all of the main ones), with same language rendering, it should normally be possible to produce output that it is 100% in the required language - although sometimes you may have to do a little work to get there. With other language translation, typically some parts of reports will not be translated, and you should expect this. In general, for example, notes are not translated. Also if you manually override sentences in narrative reports, these manual overrides are not normally translated. Where parts of a report or diagram are not translated, these parts will normally be marked distinctively (e.g. by giving them a yellow highlight colour) so that the recipient can see at a glance, which parts of the report have been left untranslated. You have the option of excluding untranslated notes, or perhaps including it but leaving it with a distinctive background colour, so that the recipient can easily see which bits are in a different language.
To learn about using Family Historian to record data in a language other than English, see How to Work in Different Languages.
To learn about producing reports in a language other than English, see How to Produce Reports in Different Languages.