The following is really just a lengthy follow-up comment on some of the issues discussed in the earlier answers that address the OP's questions. Because this comment is too long to fit in an ordinary-comment space, I'm posting it as a separate answer.
I believe the OP raises two separate issues:
- How to break up the fl-ligature in a word such as "Auflage" -- while not suppressing all fl-ligatures, let alone all f-ligatures, globally; and
- How to assure that the now-separate
l glyphs don't "collide" visually.
The other answers have already mentioned various TeX- and
babel-based methods for suppressing an fl-ligature on a case-by-case basis. A quick comment on the method that relies on inserting an "italic correction," viz.,
\/ (an "explicit kern" in TeX jargon) to break up a ligature: In my view, this method may produce a reasonable result (for some, but not necessarily) all font families, if it's used on upright-shape and normal-weight letters. Unfortunately, it tends to produce awful-looking results if the words in question are set in italic and/or bold letters. If you don't use the
selnolig package for the purpose of suppressing typographic ligatures, I would strongly recommend either employing the
"| method or the insertion of an "implicit kern", such as
For what it's worth, the selnolig package -- which, by the way, requires use of the LuaLaTeX format -- employs an entirely different approach to suppressing typographic ligatures: It inserts so-called "whatsits" during the phase of the typesetting process when LuaTeX converts various character pairs and triples (such as
fft, etc.) into single glyphs. Once the ligatures are suppressed, the "whatsits" disappear and therefore do not affect further phases, such as the hyphenation stage, of the paragraph building process.
Regarding the second issue: I think that adding a non-zero (implicit) kern between
l for words whose
fl ligature should be broken up is not ideal from a typographical point of view. This issue is particularly acute for fonts in which the
f glyph has a long "arm" -- as is the case with
Linux Libertine. While inserting a kern of sufficient width will indeed succeed in separating the
l glyphs visually, it comes at the cost of creating a noticeable (and hence distracting) visual hole in words such as "Auflage", "drauflegen", "reiflich", and so on.
In my view, a better solution involves selective replacement of the ordinary "long-arm"
f character -- if followed by
l (or any other character that might create an unsightly collision) -- with a "short-arm"
f glyph from the same font family. The good news is that quite a few Opentype font families offer such "short-armed"
f glyphs. The bad news is that it takes some work to perform this character substitution. In the next version of
selnolig, which I hope to release in the next couple of weeks, this method is implemented for the font family
EB Garamond, which features short-armed versions of both
ff. I'm currently investigating possibilities for implementing the long-arm-f to short-arm-f substitution for
Linux Libertine as well. Stay tuned...