A popular translation tool may be "Poedit", I am using that, too.
In others, "Loco translate" plug-in seems very easy for a webmaster.

 

Trial environment

WordPress4.1.1
ThemeTwenty Thirteen 1.4
Plugins OLBsystem 0.6.7
Loco translate 1.5.1
WP Multibyte Patch 2.2

 

Procedure

  1. Install "Loco transrate" and activate it.

    Loco translate

    Loco translate

  2. Open the menu "Loco translate: Manage translations".

    Loco menu

    Loco menu

  3. Click the "+New language" of "Online Lesson Booking" in the "Plugins" lists.

    Click "+New language"

    Click "+New language"

  4. Select new language.

    Select new language

    Select new language

  5. Start transrating.

    Start transrating

    Start transrating

  6. Save new transrations. But, the name of translation file has to be "olbsystem-xx_XX.mo" (or ".po"), so it's necessary to change it.

    Save new translations

    Save new translations

  7. Open a "/wp-content/plugins/online-lesson-booking-system/lang/" directory by the FTP client (Filezilla etc.)

    Open "/lang" dir

    Open "/lang" dir

  8. Change the name of translation files.

    Change the file name 1

    Change the file name 1

    Change the file name 2

    Change the file name 2

    Change the file name 3

    Change the file name 3

  9. Confirm that file name change was reflected.
    Open the menu "Loco translate: Manage transrations".

    Manage transtations

    Manage transtations

    File name was changed

    File name was changed

  10. Confirm the front end view.

    Front view ( lang=fr_FR )

    Front view ( lang=fr_FR )

 

Language chooser plug-in

The "qtranslate X" plug-in etc. may be used to change a language in a website.

qTranslate X

qTranslate X

Lang chooser widget

Lang chooser widget

Front end sidebar

Front end sidebar

 

When using "Poedit"

When using "Poedit", the procedure is as follows.

  1. The "olbsystem.pot" file is in a "/wp-content/plugins/online-lesson-booking-system/lang/" directory.
    Copy "olbsystem.pot" and name it as "olbsystem-xx_XX.po".(e.g."olbsystem-fr_FR.po")

  2. Translate "olbsystem-xx_XX.po" and save it, then it is converted to "olbsystem-xx_XX.mo".(e.g."olbsystem-fr_FR.mo")

  3. Upload " olbsystem-xx_XX.mo" to a " /wp-content/plugins/online-lesson-booking-system/lang/" directory.