First, some background to lit a light. There are some functions in PHP that may produce unexpected results in special situations. This functions are used by PW core files and in user contributed modules. Some users may have problems with file upload (pdf, zip, image, whatever) if filename has non-ascii character at the first place. For example, if user upload a file named "år.jpg" (year in Swedish language), it is expected that PW would transliterate (transform) the filename to "ar.jpg", but because of the bug in basename() PHP function, the filename would become just "r.jpg". Nevertheless the file would upload successfully. But if the filename is "привет.jpg" (Hi in Russian language) the upload would fail.
There are two ways how to handle this:
Write and use custom functions instead of builtin functions
Let end user fix that with proper locale
Option 1. is used in some CMS/CMF (like Drupal), PW (Ryan) opted for option 2. After successful login PW perform a test and warning is issued in case of test failure. It is now your responsibility to set proper system locale. The locale is a model and definition of a native-language environment. A locale defines its code sets, date and time formatting conventions, string conversions, monetary conventions, decimal formatting conventions, and collation (sort) order. Those "problematic" builtin PHP functions are locale aware, that means they work as expected, if locale is configured appropriately.
Locale can be set by the underlying operating system or in PHP. Because on shared hosting you don't usually have root access to the OS, the only option to set the locale is by using the builtin setlocale() PHP function. But, before you can use/set the locale, it must first be installed on the OS. On most unix systems at least some basic locales are already there, they are just not used. To check what locale are you currently using and what locales are available for you to use, create the file testlocale.php at the root of your website with the following content:
echo "<pre>Current locale:\n"
echo "\n\nAvailable locales:\n";
echo exec('locale -a');
Then point your browser to http://yourwebsite/testlocale.php
If your current locale is "C" (and you are on unix), then you will probably get warning after login to the PW admin. What you want to do is change the locale to something that has "UTF-8" or "utf8" in the name of the available locales. In your case you would be looking for something like "sv_SE.UTF-8" or "sv_SE.utf8". Now, there is a chance that Swedish locale is not installed. You can ask your hosting provider to install it for you or use some other UTF-8 locale. On most unix systems I have seen, at least "en_US.UTF-8" or "en_US.utf8" is installed. If even that English locale is not available, then look for "C.UTF-8" or "C.utf8". And this is what you have to use as parameter in the setlocale() call:
You could actually "stack up" multiple locales and one will eventually work:
setlocale(LC_ALL, "sv_SE.utf8", "sv_SE.UTF-8", "en_US.UTF-8", "en_US.utf8", "C.UTF-8", "C.utf8");
Now, regarding where to put that setlocale() call. If you are not using PW multi language capabilities (you don't have Languge Support module installed), then you place setlocale to /site/config.php. That's it. If Language Support module is installed (PW checks for that), you have two options:
Translate the string "C" in LanguageSupport.module for each installed language
Put setlocale() call in /site/init.php
My preferred method is option 1 and this is what PW (Ryan) recommends. See https://processwire.com/api/multi-language-support/code-i18n/ for more info on translating files. Option 1 allows you to set different locale for each of the installed language, while option 2 allows setting of one locale for all languages. PW will also provide links for the files that needs to be translated in the warning message.
Ryan just pushed an update that makes it use the locale setting on the frontend when using LanguageSupportPageNames module (until now just language was changed, but not locale). Also setLocale() and getLocale() methods are added to the $languages API var.