When TeX and LaTeX first started being widely used they largely only handled European languages out of the box, although there was some capability for using other alphabets such as Greek and Russian.
Originally, accents and accented letters were typed using control sequences or
macros such as \c{c} for ‘ç’ and \'e for ‘é’. While some people continue to
use these input methods because they can be easier to type, others wanted to be
able to use the keys on their keyboards to input such symbols directly.
Before Unicode, LaTeX provided support for many types of file encoding that
allowed text to be written in various languages natively — for example, using
the latin1 encoding French users could write ‘déjà vu’ and LaTeX would
internally translate the accented letters into TeX commands to produce the
correct output.
This approach is still in use in modern LaTeX when using the pdflatex engine.
By default all files are assumed to be Unicode (UTF-8 encoded) unless otherwise
specified. Although this engine is limited to 8-bit fonts, most European
languages can be supported.
Font selection with pdflatex uses the robust LaTeX font selection scheme, and
nowadays there are many fonts ready-to-use in a standard LaTeX distribution. For
example, the TeX Gyre fonts are a suite of high-quality fonts based on common
fonts that most people are familiar with such as Times, Helvetica, Palatino, and
others. To load these fonts, it is as simple as loading a package with the
appropriate name. For a Times lookalike, the TeX Gyre name is Termes:
\usepackage{tgtermes}
For pdflatex, most fonts are accessible through packages. You can have a look
at The LaTeX Font Catalogue or the
CTAN page on the ‘Font’ topic to see some
options. You can also search on the Internet for the font you want, and look
for a pdflatex-compatible package version. If you want to use a proprietary
font, you can search for a suitable clone, which for most applications is
similar enough to the original.
As pdflatex is limited to 8-bit file encodings and 8-bit fonts, it cannot
natively use modern OpenType fonts and easily switch between multiple languages
that use different alphabets (or scripts, to use the technical term). There are
two replacements for pdfTeX that natively use Unicode input and modern fonts:
XeTeX and LuaTeX. For LaTeX, these are typically invoked in your editor using
the engines xelatex and lualatex respectively.
In these engines, font selection is performed by the fontspec package, and for
simple documents can look as easy as:
\usepackage{fontspec}
\setmainfont{texgyretermes-regular.otf}
This selects the TeX Gyre Termes font, as in the pdflatex example above.
Notably, this approach works for any OpenType font. Some fonts available for
pdflatex are also available to xelatex and lualatex through their
respective packages as well, or by loading any font you have installed on your
computer by using fontspec as shown above.
The LaTeX Font Catalogue also shows fonts
with OpenType formats available, so you can use that as a resource for looking
up fonts, as well as the CTAN page mentioned
earlier.
Having selected a font, input can now be typed directly in plain Unicode into a source document. Here is an example showing some Latin and Greek letters as well as some CJK ideographs:
\documentclass{article}
\usepackage{fontspec}
\setmainfont{texgyretermes-regular.otf}
\newfontfamily\cjkfont{FandolSong-Regular.otf}
\begin{document}
ABC → αβγ → {\cjkfont 你好}
\end{document}
When switching between languages it is usually important to also
change things like hyphenation patterns and so on, and the babel and
polyglossia packages both provide robust features to do this.