Pootle supports hosting monolingual files natively since Pootle 2.1. Monolingual files need special attention in order to provide translators with good workflow and to assist to perform good translation. Read more in the localisation guide.
The main difference between monolingual and bilingual projects in Pootle is that for monolingual projects a translation template is required. Pootle cannot meaningfully import strings from monolingual files unless the original text is present.
Either the source language or the special templates language must be added to the project and their files uploaded before other languages are added. Files found in either will be considered template files (in the case where both templates and source language exist templates will be used).
What users will see when translating monolingual file is a matching between strings in the templates file and strings in the target language files. The matching is format specific (for example in subtitles the matching is based on timestamps, for Java properties it is based on keys, etc.)
While Pootle supports uploading translations in the monolingual format this should be limited to importing old translations. Users who want to translate offline should download the XLIFF version (from the translate tab).
When tracking monolingual files with version control, if the file structure changes (e.g. new strings are added) then source files must be updated first.
Apart from these considerations monolingual projects will feel and behave the same as bilingual projects, all of Pootle's features are available to administrators and translators.
You can still use the format converters from the Translate Toolkit to host these monolingual file formats as a Gettext PO project. This has the advantage that files in version control always have the source and target strings together and you are able to integrate with external PO tools.
Currently supported formats include: