The taginfo user interface allows you to switch to your favourite language. By default the language configured in your browser is used, but there is also a drop-down box that allows you to change it. If taginfo doesn't know about your language it uses English.
Help by translating the web site into your language
Nearly every text in the taginfo web interface can be translated into any language. All texts are in the git repository in one directory with one file per language. The format of the file should be easy to understand. There is a hierachy of labels ending in a colon (:) and after them the text. Multiline texts have a bar (|) after the label and the text in the following lines.
To view the translation status of currently supported languages see the taginfo i18n page.
If you only want to add a few translations or change some, the easiest way is to go to the github directory, click on the language file, then click "Edit". You can edit the file directly in the browser and after you are finished send a pull request with your changes. You'll need a Github account for that.
If you want to translate a larger portion it is probably best if you fork the project on github and start translating. Once you have finished all or part of the translation, check it in and send a pull request. If you can't work with github, just send me the language.yml file, but thats more difficult in the long run if changes are needed etc.
If possible you should use the English (en.yml) or German (de.yml) language versions to start translating from, because they are complete and always updated when the software is updated. Please take care to translate the OSM technical terms (such as "node", "tag", etc.) in the same way as they are generally used in your OSM community. Dictionary translations are not always right for these special terms.
You do not have to translate everything, if it is too much work. The labels are ordered roughly beginning with the more important texts so just start at the top and do as much as you can. If a text is not translated taginfo will fall back on the English text automatically. Do not put any text in your language file other than the translated text.
In the git repository there is a handy little script that checks whether you have all texts by comparing to the English text file. For instance to compare the English with the Italian text you run it as follows:
bin/check-translations.rb web/i18n it
To check your translation you can install taginfo, but you don't have to do that, if it seems to complicated. When you send us the files, we'll updated the main taginfo site and you can see your translations there and update them if needed.