Welsh has a wide variety of plural forms that include internal vowel changes, suffixes, internal vowel changes and suffixes, et.c. Examples, bachgen (sing) bechgyn (pl) - boy; pysg (pl), pysgod (sing) - fish; cyfaill (sing), cyfeillion (pl) - friend;

German? Like das haus, die häuser/der Mann, die Männer (a house, houses/ a man, men)? I'm a beginner in German, but they usually make plural by adding suffixes or some nouns change their stem by transforming an 'a', 'o' or 'u' in the stem to their respective umlauts 'ä', 'ö', or 'ü'.

Look at the English language: man - men, mouse - mice etc.

... and look at the English irregular verbs or Slavic verbs (perfect and imperfect aspects).  

I can see that many languages do have morphemic cases with internal vowel modification, as in English 'man>men' or German 'haus>häuser'; but what I referred to in the question is the Arabic regular plural patterns, not irregular plural forms that seem to be accidental in these European languages. In Arabic, the pattern ['af'aal], for instance, is applicable to many tri-consonantal roots to yield plurals like ['awlaad] 'boys', ['awraaq] 'leaves', ['aTfaal] 'children', ['aHlaam] 'dreams', etc...    

The irregular plurals can be broken into classes, but I am travelling and don't have access to my reference materials.  Also, my Welsh references are language learning materials rather than linguistic analyses.  However, I will take a look when I return home.

@ Zoubir

Who decides about what has to be called "regular" and what has to be called "irregular"? 

Well, this is obvious ! What's regular is the rule; e.g., all English nouns have an {-s} in the plural, except for a few exceptions like 'mouse' > 'mice'... same for {-ed}, the past morpheme in regular verbs. 

In German we have (as far as I see) 13 regular(?) plural-morphemes (-⩝- means vowel-change): -e, -⩝-e, -er, -⩝-er, -n, -en, -s, -∅, -⩝-∅, -ien (Indiz – Indizien), -ten (Bau – Bauten), ⩝-en (Werkstatt ­– Werkstätten), ⩝-te (Zeitlauf – Zeitläufte). The last 4 are rare. And we have no rule, that tells us, which singular takes which plural-morpheme. (Do you have such a rule in your language?) Additional we have a number of special plural-morphemes in loanwords (one example: Museum – Museen) and the suffix -se which exists only in writing (Bus – Busse).

The difference is that in Standard Arabic, there are a great number of regular patterns used to pluralize nouns and adjectives; the patterns differ according to the type of noun patterns, and many have got a form called 'broken plura'; e.g  the plural of [kitaab] 'book' (pattern {fi3aal}) is [kutub] ({fu3ul}). Read my article 'The Broken Plural Morphological System in Arabic' (scroll to top, or get it here using this link).

http://www.academia.edu/3067643/The_Broken_Plural_Morphological_System_in_Arabic_A_Challenge_to_Natural_Language_Processing_Models

It seems to me that the Arabic broken plural morphological system can be compared with the tense morphological system of the Germanic languages (take – took – token).

Honestly, I can't see how, for the Arabic broken plural system is based on regular rules. Have you read my article?

The morphological system of the strong verbs of the Germanic languages (sing – sang – sung – song) was origionally entirely regular. The seven classes can still be recognized in the German language.