Assume one wants to cite a paper written in a different language than the article citing said paper (e.g., ones own article is written in English, but the source is French).
How to properly handle citation in such a case?
Do I cite the original (French) title or a translated title? If I use a translated title, people might have a hard time to find the source; if the original (French) title is cited, people might have a hard time understanding it.
You cite the original title, for the very reason you gave: to enable readers to find the original title. If your work is subject to guidelines, you should also check those.
Usually, there is no point in giving a translation of the title as it does not contain any relevant information for the reader. Many citation styles do not mention the title of papers at all. I see two exceptions though:
If a translation of the cited work into English exists (but you worked with the French original), you can mention it in addition to the French title, e.g. with:
[actual citation] (translated into English under the title [translated title])
If the title allows the reader to estimate what the source contains and whether they want to read it at all. In this case, you should arguably change your text such that it states in what way the citation is relevant for your work.
It actually depends on the citation style. It is particularly well documented in the APA Style (see Citing Translated Sources in APA Style):
For example, here’s how you would cite the original French edition of a work by Piaget (note that an English translation of the title is included in brackets):
Piaget, J. (1966). La psychologie de l’enfant [The psychology of the child]. Paris, France: Presses Universitaires de France.
and:
Here’s another example, from a German journal. Again, brackets contain an English translation of the work’s title (the article, not the journal).
Janzen, G., & Hawlik, M. (2005). Orientierung im Raum: Befunde zu Entscheidungspunkten [Orientation in space: Findings about decision points]. Zeitschrift für Psychologie, 213(4), 179–186. doi:10.1026/0044-3409.213.4.179
If the reference uses a foreign alphabet, be sure to also check the transliteration requirements.
It is common to either use a combination of the original title and a translated title. The exact form for punctuation has to be adapted to the specific journal. I use an example from "my" journal (Instructions for Authors):
Author name(s), year. Title in original language (if possible) [Title translated into English]. Publication name in original language (if possible) [Publication name translated into English]. Volume/issue/page information (according to type of publication). [In ‘language’]
Example:
Krenke, A.N. and Khodakov, V.G., 1966. O svyasi povercknostnogo tayaniya lednikov s temperaturoy vozdukha [On the relationship between melt of glaciers and air temperature]. Materialy Glyatsiologicheskikh Issledovaniy [Data of Glaciological Studies], 12. 153–163. [In Russian]
If the original is in a language written with different characters such as Russian, Chinese or Japanese to mention a few then one can also use a shorter form such as (again using the example above:
Krenke, A.N. and Khodakov, V.G., 1966. On the relationship between melt of glaciers and air temperature. Data of Glaciological Studies, 12. 153–163. [In Russian]
In the Russian case there sometimes exist a transcription with Latin letters but not always.
I will add that many journals in my field do not translate titles in for major languages such as Spanish, French and German since it is assumed that everyone has some grasp of languages. This is of course a notion of the past since the "western focus" is, and has not been for long, a useful perspective.
For references in other languages (non-English), I use titles translated into English if the translation by the author is given in the paper (usually together with an English abstract). In such a case, a remark should be put at the end in brackets, eg. (in French). The use of translated titles should also be recommended for papers written in non-Latin scripts, eg. Chinese.
