You can translate the FS Poster plugin easily using any translation plugin. The instruction includes two ways, and the process is the same for all other tools.
Please note that some warning messages that come from social networks cannot be translated.
Every FS Poster release ships an up-to-date translation template, fs-poster.pot, with all the texts of the plugin.
Make sure the language you selected matches your WordPress website language. For example, if your WordPress language is English (United States), but you selected English (Great Britain), it will not work.
- Download and install the Poedit software on your computer;
- Download the translation template file. In standard WordPress installations, it is located in the “wp-content/plugins/fs-poster/languages/fs-poster.pot”. If you changed the folder names or moved them to another place, you need to search and find the fs-poster.pot file;

- Open the tool and select the “Create new translation.”

- Next, you should select the template file which you downloaded in the first step. You will be prompted to select a language. You should select the language that you use for your WordPress website. Otherwise, the translation file will not be used;

- And start translating;

- When you finish the translation process, click the Validate and then the Save buttons;

- Default filename will be the only language name, as shown in the image below. You need to add the prefix the “fs-poster-” to it. For example:
fs-poster-en_US.po
fs-poster- fr_FR.po

- And click the Save button.
- Upload the saved .mo and .po files to wp-content/languages/plugins/. WordPress loads translations from this folder first, and plugin updates don't touch it. Files in wp-content/plugins/fs-poster/languages are replaced when FS Poster updates.
- After installing the Loco translate plugin to your website, go to the Loco translate > Plugins from the WordPress admin sidebar;

- Click the FS Poster plugin on the plugins list. Then click the New language button;

- Select a language for the new translation. If your language is not in the WordPress language list, you can also add a custom language. You should select the language that you use for your WordPress website. Otherwise, the translation file will not be used. Choose the System location (languages/plugins/fs-poster-{locale}.po) or Custom. Don't choose the Author location, the plugin's own folder, because updates replace it. And click the Start translating button;

- Select the sentence\word from the list and type the translation in the dedicated box. When you finish the translation process, click the Save button.
New versions add new texts. To translate them in Poedit, open your .po file, choose Translation > Update from POT file and pick the new fs-poster.pot. In Loco Translate, open your language and click Sync.